Browsing BigGym.co.jp, yet another online store for Japanese gay men, I discovered a wonderful archive of cover scans for Samson magazine, which is where I got the above image. Doing some web research, it turns out that Samson is an adult magazine focusing on older Japanese men. As far as I can tell from a quick browse of the official website, the magazine features adult photospreads and manga. But I'm mostly interested in the covers, which I believe are illustrated by an artist named Pontaro.
Here's some NSFW links for Samson magazine's official website:
Cover index from June 1998 to December 2003. You can click through to see photos and larger cover scans for all issues from 2002 and 2003.
Cover index for 2004. You can click through to see more photos for each issue.