Japan is quite a bit stricter than most countries on qualifications for employment visas. Many jobs require master's degree plus TEFL/CELTA certification and experience teaching ESL, not teaching some other subject. You must be a native speaker of English from an English-speaking country (UK, US, Canada, Australia, NZ). Minimum of bachelor's degree in English, Linguistics, Applied Linguistics, or related field. Master's degree often required, always preferred.
TEFL experience in several countries