Update:
[24/2/2021]Updated the new plan of IIJmio.

We have summarized the new cheap plans from each MNO / MVNO starting around the spring of 2021 of japan.

The comparison targets are major MNOs / MVNOs, with capacities of 20GB (or more) and 3GB.

Data capacity: 20GB plan comparison

First, see below for the comparison results at 20GB.

Regarding the content of the comparison, the part that seems to be particularly superior is shown in “green”, and the part that seems to be inferior is shown in “red”.

Best plan: Rakuten Mobile
Looking at this plan comparison table, regarding 20GB (or more), Rakuten Mobile seems to have many advantages, but in terms of the service area that can not be seen in the plan, it has the disadvantage that it is narrower than other MNOs. Therefore, it is better to confirm the area where you live in advance when considering the plan.
If the range of action is mainly within the Rakuten area, this plan may be the best.
However, Rakuten Mobile plans to start a service using satellites in 2023, and it seems that the out-of-service area will be eliminated by that time.

Lowest price: Rakuten Mobile, Japan Communications, (IIJmio)
Looking at the monthly fee, Rakuten Mobile and Japan Communication are about 1000 yen cheaper than other companies.(IIJmio is the lowest price, but the free call part is not clear, so it is for reference only)
In addition, even when the capacity is exceeded, Rakuten Mobile will have unlimited capacity, up to a monthly charge of 3278(taxed price) yen. Regarding Japan Communication, the additional capacity charge after exceeding 20GB and the call charge after exceeding 20GB are half the price compared to other companies, making it a cheap plan.

One of the disadvantages of Japan Communication is that it is a fee-based system, and although it costs money, I think it will pay off if it is operated for half a year.
Also, the part that has been considered a disadvantage in the part that does not have a store may be faded by the appearance of online-only plans such as ahamo and povo.

IIJmio is the lowest price, but since the new plan site does not include explanations such as voice call charges, it seems that there will be no free calls, but those who mainly use data will be charged additional capacity. Isn’t it good because the flat rate is the cheapest?

Plans that don’t feel much benefit: Y! Mobile
The highest monthly charge is Y! Mobile (10 minutes free call is added for comparison with other companies). Even with a family discount, the plan is 3740 yen(taxed price), which is higher than other companies. The reason why it is cheaper than other companies is that if you do not add free calls for 10 minutes, it will be 2970 yen(taxed price) and you can finally compete with other companies, but you need to be careful because all calls will be charged.
Because this fee is expensive, the parts that are superior to others are “in-store support” and “carrier mail.” If “support at the store” is important, Rakuten Mobile also provides over-the-counter support, and with regard to “carrier mail”, there are now free mails, etc., so I do not feel the merit of the amount.
As of January 2021, sub-brands, including UQ mobile, may have less competitive plans.

Comprehensively secure plan: ahamo, povo, Softbank on LINE
At this stage, the ones that are generally reassuring are ahamo, povo, and Softbank on LINE, which use the stable network of NTT docomo, KDDI, and Softbank. If you are not unfamiliar with online contracts and can fit within 20GB for everyday use, this plan will be a relief.

Data capacity: 3GB plan comparison

Next is a comparison with 3GB (less than). Since the 3GB plan has only Rakuten mobile as the MNO main brand, it is a comparison between Rakuten mobile and the MNO sub-brand, MVNO.

Again, the trend is the same as 20GB,
Best plan: Rakuten Mobile
In terms of amount of money, Rakuten mobile, which costs 980 yen for 3GB, is the cheapest, and even more surprisingly, if the used capacity is less than 1GB, it is “0 yen”.
This newly announced plan can be said to be a very centripetal plan.

Plan with comprehensive peace of mind in terms of area and price: Japan Communications
Japan Communication’s plan is probably the best plan with a comprehensive sense of security in terms of area coverage and charges.

A low-capacity plan with a safe area: IIJmio
It is IIJmio that has the same area coverage rate as major MNOs and has a low dose (2GB, 4GB, etc.) cheapness.
However, please note that it seems that free calls etc. are not included for voice, although it is also described for 20GB people.

In addition, UQ mobile and Y! Mobile seem to be in demand only for those who require shop support and are outside the Rakuten mobile area.

Attention of cheap plan

As of February 5, 2021, I checked the attention of the cheap plan using Google Trends.

Looking at the results, the degree of attention is as follows.
1st：ahamo
2nd：povo
3rd：Rakuten Unlimited
4th：Japan cominucations(MVNO)
5th：softbank on LINE

Regarding 5th place softbank on LINE, it seems that the attention is a little inferior to others.