Penalty for not wearing a bicycle helmet

What is the current understanding over bicycle helmets, particularly in small, unbusy roads? Do the ordinary Japanese wear bicycle helmets? Is there a penalty for not doing so?

There is currently no penalty. Depending on the prefecture of Japan the helmet-wearing percentage in 2023 was between 2 and 60 percent:
https://uub.jp/pdr/q/helmet.html

It�fs a �gduty�h to wear one, but not a requirement.
From pure observation I�fd say casual cyclists in the city unless they are children, policemen or otherwise made by their uniform to wear a helmet rarely wear one. Road cyclist in the other hand do nearly all wear one.
So essentially decide for yourself. If you wear one , no one will think your are strange.

Thank you Uji, LikeBike. This is useful to know.

The English media report it is a requirement. My understanding is that the Japanese wording is an encouragement. But, there is no penalty.