At some point after you arrive, and every time you re-contract, your contracting organization will present you with a small certificate that officially recognizes you as one of their employees. It should have your name, details about what you will do, the date when your contract begins, and the date when your contract ends. This is yourJinji Tsuuchisho (人事通知書), and is your formal notice of employment, so try not to lose it. If you want to stay in Japan after JET ends and keep working, you will probably need this, as it can serve in lieu of a Taishoku Shomeisho (退職証明書), or “proof of retirement.”
SHS ALTS: First-years receive their Jinji Tsuuchisho at Orientation (the prefectural conference in August in Kobe).
If you recontract, you should get your new Jinji Tsuuchisho from your school.