Question

why is Japan is called developed country.

Answer 1

Developed is a term used to describe countries who are up to date, becoming up to date, or are ahead of what we deem to be the current standards of living.
Of which, Japan is all three categories.
There are some cases where they are quite advanced, and other cases where they are quite 

Answer 2

Japan is a developed country because of their people hard work
They have introduce lots of Technology have high power of developing new new things with first class.

it has highest economic power which is highly developed and market oriented economy.