Basically, as long as you are holding a degree or in your last year of undergraduate studies, you are eligible. Based on your application forms, you ll be shortlisted to take an English test, n if you happen to pass it, an interview will follow. Although its stated that a 1st Class or 2nd Upper degree is ideal, it shouldnt be much of a hindrance even if your CPA is below 3.
And of course, they listed field of studies which are eligible for application. I would advise you to check the embassy of Japan's website in your country, usually ads about this scholarship will be shown there.
Good luck!!