Bangkok is a heaven for eating. You'll find many international restaurants throughout city. If you're missing your homeland cuisine, there is at least one restaurant from your country waiting for you for sure. The Japanese are lucky. Japanese food is quite popular in Thailand as you can see from franchise restaurants like "Fuji", "Zen", "Yayoi", "Oishi" etc.
Apart from those famous restaurants, I recommend another one called "Giant yakiniku". It is an all-you-can-eat restaurant with a fix price of 330 Baht per person including drinks and service charge. The beef lovers should try it. There are more than 10 kinds of meat for you to choose along with salad and seaweed.
How to go?
Take BTS skytrain to Chid Lom Station. Walk up north on Chid Lom Road along the Central department store for about 300 meters. You'll see the restaurant on the second floor.