The Japanese Constitution protects the freedom of choice of employment. Therefore, within limits, people should be able to work for whatever company they want. However, Non-compete clauses in Japan are very common in contracts.
Japanese law does recognize the validity of non-compete clauses to certain extent. If the non-compete clause is reasonable in the length of time, the geographical area and the specific industry, then a court might uphold the non-compete clause as valid. In some situations, sufficient compensation must also be provided.
Some contracts include non-compete clauses which are overly broad, which may not be enforceable under Japanese law, so it is important to know the limitations on the enforceability of such clauses.