There are a lot of laws and regulations like Labor Standards Act and Labor Contract Act, etc. about labor contracts and working conditions.
Changes in working conditions that are disadvantageous to workers
The agreement from individual workers is required to change working conditions such as wages, working hours, vacations, and welfare programs that could potentially disadvantage employees. Generally, Any changes made are required to be agreed on paper. Changes are possible through changing Employment Regulations but important items like wage reduction may not be permitted.
Fixed-term working contracts
The fixed-term employment contract is renewed if it is not objectively rational and is not considered to be appropriate according to social conventions and any of the following applies.
It has been repeatedly renewed in the past, and the termination of employment can be equated with the dismissal.
There are reasonable reasons to expect a worker renew when the contract expires.
Changing a fixed term contract to a non fixed term contract
When it has been more than 5 years in total after renewing a fixed-term employment contract, it can be changed to a non fixed employment contract by the request of the worker. The contract type such as contract employees, part-timers, part-time job status, and temporary employees are irrelevent in the renewal process.
★ In Japan, because the constitutional guaranteed on "freedom of choice of occupation" and "freedom of business" are restricted, if the restrictions on competition exceed the reasonable range, and employee's freedom to choose their occupation are unfairly detained and their lives are threaten, the restrictions may be deemed invalid, contrary to public order and morals. The criterion for judgement is whether the employee is using the general technology obtained by the business, or whether the employee is in an important position such as a member of the board and is an act of collecting customers and know-how.