A white post is a garbage bin for pornographic magazines so that kids cannot find them on a street or in an open garbage bin. It stands here since I was a child and I always doubt if this is actually used (the location is right next to the gate of the station, and who would abruptly take a pornographic magazine from a bag and throw it in this post without not being noticed by anyone?).

A quick search shows a Wikipedia article about the white post (in Japanese and in German) and according to the article, white posts were installed so that adults will not bring the magazines back home and read by children. Nevertheless, because of online contents, the number of white posts are decreasing.