There are very few recognized training courses for English teachers in Japan. There are MA courses such as the University of Birmingham MAs in TEFL/TESL and Applied Linguistics and the University of Sheffield MA in Advanced Japanese Studies, but there is very little for those who want take a shorter course that leads to a certificate that is widely recognized.
In many countries there are one-month intensive training courses that lead to qualifications that are valued by language schools, but there is little demand for this kind of course in Japan. Most teachers cannot just come to Japan and take a one-month course before looking for work, and hardly anybody can take one month off from work to get better qualified.
This is why Oxford University Press and David English House started the one-day certificate course in teaching Japanese students about eight years ago. Just about everybody can take a Sunday off to attend a course and can spare time to write up the assignment afterwards. This certificate course has built up a very high reputation over the years and is highly recommended for teachers of teenagers and adults. The courses will be held in Tokyo, Osaka and Nagoya on Sundays in June. You can see full information on the Oxford University Press website.