NYC alone buys about 3,000 manhole covers a year (from the NYT article linked downthread) for just electricity and steam operations (not including sewer, gas, or water).
Japan has around 14x as many people and is substantially more urbanized, so it's not inconceivable that they run production daily, especially if they're selling to other markets.
I would imagine they do that a lot. They seem to have YKK'd the manufacturing of manhole covers and I can't imagine that anyone else could do it better or cheaper.
Edit: if anyone didn't get the YKK reference, look at any zipper you own. Chances are it says 'YKK' on it. YKK of Japan has perfected zipper-making and dominates the market.
Some of these seem to be new installs since they're also making the mating ring that goes around it. That mating ring is embedded in concrete.
Also, since they seem to have the concept of painted manhole covers, I would imagine that they would replace slightly more often than we do to commemorate special events.
Manhole cover theft is a problem too (as part of general scrap metal theft as the cost of iron and other metals has risen) but I would imagine it's less of a problem in a society like Japan.
Also, do manhole covers really need to be replaced that often? That's a lot of manhole covers. Surely they don't make that many every day.