Most jobs teaching English in Japan are adult education positions. That is, the aim is to teach English to Japanese adults, most often in a business setting. These jobs rarely require the "teacher" to possess traditional teaching credentials.
Other companies offer positions teaching children, but the requirements are often more stringent for these positions and include traditional teaching credentials above and beyond the requirements for adult education positions.
The first standard requirement is commitment. Individuals who commit to teaching in Japan commit to staying in Japan. Often, the commitment is for a full calendar year, and there is no built-in trip "back home" during that year for the teacher.
A second type of commitment is to the job of teaching itself. Teaching, especially teaching a second language, is a time-consuming job. Many additional hours outside the classroom are required of a teacher for planning, grading and other administrative tasks.
The second requirement for teaching in Japan is that the potential teacher must have a strong command of the English language and particularly of business English. Good grammar, spelling and composition skills are a must.
A teacher's communication skills must be above par as well. Communication with students who are not skilled speakers of the teacher's language is challenging. Teachers must be able to communicate with people who may not be able to successfully communicate with them.
Few, if any, companies who hire teachers will hire an individual without a college degree. Degrees in English and business are recommended for those who want to teach in Japan. Some experience teaching English as a foreign language is a bonus as well.
In addition to the formal and company requirements, there are a few practical requirements for teachers in Japan. Flexibility and respect for diversity are critical for anyone who wants to teach in a foreign country. Good physical health is a benefit--most travel around Japanese cities is done either on foot or in public transportation.