Beware

Things you won't know about until you are over your head:

Taxes: You will have to pay taxes beginning the second year you are in Japan. They are increasing to pay for the huge numbers of aging senior population.

Health Insurance: by law, companies are supposed to pay for you as their worker, but the govt turns a blind eye to it.

Job Discrimination:
  Any foreigner married to a Japanese national wIll get higher priority than, even if you have worked at a company longer than them. If single, the company believes you can up and just leave, but the married guy will stay because he is tied down. I lost my job to a new married guy who just arrived, after I had worked for a company for 4 years. The company was aegit and descent company, but got cut throat and self serving...the largest business English company in Japan. Loyalty and control is all that matters to companies in Japan. If they control you, they make money automatically. All compliments about being a great worker is just to keep you to stay around.

Time of Stay: 
  Most English teachers only stay for 6months even if they sign up for a 1 year contract. They are hit with culture shock, major working culture differences AMD bail out back home. Saw this many times.
Here is reality from my own direct experience: imagine 100 teachers go to Japan the first year, 80 will leave for home after that first year; 10 more will leave tphe second year; 5 more the 3rd year, only 3-5 will stay for more than 3 years. I stayed for 5 years and I knew every English teacher in a city 5,000,000 people I had been there so long. I knew all of the oldtimers and university profs that got promoted by default having lived there for so long....I was almost a univ prof myself and taught everyone type of class in town.