The pads on my glasses were getting really old.
The optician used to replace them periodically free of charge, but they don't seem to want to do that any more.
I found these replacements on Amazon.co.jp. I paid 550 yen for two pairs of these. They seem to be a standard size because they could easily be fitted.
The glasses are looking a lot better now:
Unfortunately I don't have a "before" picture because I left my camera switched on and plugged into the computer today, and the battery ran out. It (Nikon D40) doesn't have the capability to take power from USB. However, the nose pads had really got disgusting. The glasses had spent a whole night in the Pacific Ocean, among other things, since the last time the pads were replaced.
For the replacement, I used this screwdriver:
One tip about doing these jobs with glasses is that it's better not to use the small screwdrivers from 100 yen shops since they're not properly made, and they're prone to bend, break, and damage the screws. This 000 screwdriver only cost only a few hundred yen at Keiyo D2 (Daily Discount).