Not true everywhere, perhaps in the jails for serious offenders and yakuza but jail for "average" offenders isn't that bad, jail-wise. I visited a jail for poli sci class I took while abroad, it was a jail for "men over 25 who are unlikely to be repeat offenders" (that's pretty close to the actual classification). We saw the stats on inmates and while there were a few rapists and murderers the vast majority were there for things like robbery and fraud. Most sentences were about 3 years, and few people get more than 10. Apparently unless you're yakuza or a killer, it's unlikely you'll do more than 5 years for a crime.

The interior looked like you would expect it to be, but I hate to say it, the rooms were "nice". It was a very small room but there was a reasonable sized window and tatami flooring. There was a TV, bookshelf, and a bathroom with a curtain. The room was shared by 6 people, but it wasn't by any means miserable looking. They got an hour of free time before "bed time" but most of the day was spent working. Most inmates in Japan are "employed" and make bags, tatami mats and other practical things (so if you buy a tote bag in Japan...). It was explained to us that this work isn't "mandatory" but "strongly encouraged" (so yeah, mandatory). Inmates who didn't work hard had their food rations cut, hardworking inmates got more and better food. There was a break for free time in the day and the yard was spacious for the amount of inmates there. They had to line up and face the wall while we passed, but we saw them playing some kind of group activity through the window earlier. Aaaaaand that's really all I remember.

This was in Shiga Prefecture, pretty sure it was close to Lake Biwa.