Japanese food is very healthy, right ?

Yes and no depending on what you choose. Japanese food is acknowledged as a very healthy food but some contains a lot of sugar. Let me give you a few examples. To make the most famous and popular Japanese food Sushi, you need to make sushi rice. To make sushi rice you need to make sushi vinegar and a lot of sugar is added in the vinegar. So sushi is combination of carb (rice) and carb (sugar) with a little bit of protein(fish). Another example is teriyaki. Teriyaki sauce often contains a lot of carb, either sugar or honey, and sometimes thickened with starch which is another carb. So sugar is the reason those two dishes became very popular outside of Japan. People are addicted to them because of the sugar. But I think sugar itself is not a bad thing. It's us eating too much of it. Eating sushi once in two weeks won't hurt you but eating it every other day... you can guess the result.
Good news is, there are a lot of very healthy Japanese food too ! Nabe (Japanese hot pot) would be a very good example. Loads of vegetables and as much protein as you like, with less carb. Several years ago I lost nearly 20kg by replacing food that contains a lot of carb with Nabe.
Conclusion : Choose how much/how often of what to eat carefully !