This is a good reference which tells you about 5x most common laws
that you may un-intentionally step on when importing your goods to Japan.

違反は罰金100万円?輸入者時に犯しやすい法律 BEST5

The link is in Japanese only.
Here is the abstract of 5 laws.

1) Electrical Appliance and Material Safety Law
For anything that plugs into the power point on the wall.

2) Food Sanitation Act
Things like dishes, forks, spoons, children’s toys that may
be in the mouths.

3) Pharmaceutical Affairs Law
Prescribed Medicines, off-counter drug store products, baby powers,
shampoo, cosmetic products, eye glasses, thermometer etc.
Please remember the item you try to import may not be classified as
medicine in your country, but may be it is in Japan.

4) Radio Act
Transceiver, Wi-Fi products, Blue-tooth etc

5) Measurement Law
Everything must have scales/units that are commonly used in Japan.
For example, items which only have inches, feet, yards are not allowed.