I agree with fluffy hamster...CELTA is an initial qualification, to prepare ELT teachers for the classroom, that usually takes about four weeks to complete. A CELTA that has been issued by an unaccredited institution will probably be rejected by any good employer. (Unfortuantely, there are many cowboy institutions at this level).
By 'law', you are not allowed to take the DELTA until you have a CELTA and/or completed two years of ELT teaching. This course will cover in greater depth what you are expected to know methodologically as an ELT teacher.
The next step would be applying to a university to get - a postgraduate certificate in applied linguistics or TESOL; a postgraduate diploma, or a M.A or M.Ed. On these courses, you begin to look at things like syllabus development, sociolinguistics, test validity and reliability, second language acquisition, phonetics, research methodology, material development, etc.
Admittedly biased, but I would recommend Exeter University in terms of the teaching quality and support. You can check the Guardian, BBC or TES' league tables for further information.
I am surprised to read that you will get a promotion or a raise if you have a Masters in Japan: as an increasingly popular trend within the MEXT infrastructure is to dispose of 'gaijin' teachers after three or five years, whether they are qualified, brilliant or not.
I am equally doubtful that anyone in your local kyouikuinkai even knows what a CELTA or DELTA is. Why not have tete-a-tete with them about this before applying to do any of the above courses.

Peace & good luck
Lottie