I don't think it is illegal to own CP in japan. But publishing / distributing it.
Last time I was in Tokyo, there was still 2nd hand book stores selling Alice Club. It seemed moderately priced (c. 40-50 euros per 200 page book?), and many vintage bondage seemed to be much more "rare" and valued. Of course it's not anymore something what it used to be when it officially was in newstands. I recall published boasted with sales of 25000 or 50000 per issue, which was bi-monthly. I don't think people over there consider it that extreme. It's not like it would be sex pics. More like pre-junior idol, without need of bikinis.
Since, I wouldn't recommend anyone to try export them from Japan and see can they justify it with "nudism", "beach photos", "artistic nudes" and all that, and fact that it's mosaic/black spot censored anyways. There was also Trevor Brown stuff in issues.
Perhaps better and safer targets of hunting from Japan would be Ultra Negative / Too Negative magazines (/books).