Regarding this methods, Okadasan advised me ! (Thank you so much ! )
Please see below. It is standard position bulb. There are many LED bulbs. But the type of socket is usually "wedge type".
So I replaced the standard sockets with the "wedge type " sockets. Regarding the following photo, it is the "wedge type" sockets with cables. I used it.
The replacement is very easy. I cut the existing sockets and connected the existing cables to the cables of "wedge type" sockets. This socket is covered with rubber. So fortunately, the size of this socket is similar to the existing hole and can be fit.
As I explained, there are many "wedge type" LED bulbs in the stores. So I tried several lights. Finally, I got hand made LED lights. (5mm LED, 6 pcs per 1 socket. ) It is very good brightness, I think.
LEDs and "wedge type" socket with cable.
I think that photo is very difficult. So maybe, you cannot grasp actual brightness.. Anyway, please see below.
If you replace, you can use many types' LEDs !! :D