For those who don't read Japanese, this sign says "Warning: Perverts lurking."
This sort of 'pervert warning' sign can be spotted throughout Japan. Don't know what it is with Japan and pervs but a lot of it seems to happen on trains too which has lead to women only train carriages (read how my colleague got groped in that post).
I didn't see such signs when I was living back in the UK although a few of my Japanese friends encountered a flasher on the underground...
I suspect each country has its share of pervs. Maybe they should be given a free supply of Tenga so that they can keep their hands to themselves.