According to AmusingPlanet: “The art of manhole covers has now reached the point of a national obsession in Japan with numerous municipal departments competing against each other in the pursuit of the perfect manhole cover. The designs are manufactured by a municipal foundry where they are cast and created. The city or council will submit ideas and the symbol of choice to the foundry and their in-house designers will then create a design based on these specifications, going back and forth until the design is approved. The foundry will then cast a prototype before doing the final cast. These manhole covers are made of metal, as opposed to European manhole covers, which are typically constructed of pre-cast concrete. After the covers have been cast the carved wooden masters are saved in an enormous central library.”
Flickr member MRSY has a fascinating set of Japanese manhole covers. There’s also an awesome Flickr group dedicated to Japanese manhole covers.