The working hours are free. Legally you are free to choose when to work, even how much.
Note by Martin Heidegger: To my knowledge this is the most important point because legally japanese companies have to pay overtime if it occurs and have to specify working hours. As such startups, and many media companies, use outsourcing contracts to work around labour laws that would limit them to specific working hours.
You can legally not be asked to work outside of your profession as specified in the contract. Refusing to do work that is not in your contract is no ground for annullment of the contract.
A outsourcing contract needs to have a time limit. The advantage of an outsourcing contract is that you have to renegotiate your contract after the period. Since japanese companies tend to avoid conflict this means that without raising eyebrows you can leave or get a bigger salary.
You can work at more than one company at a time without asking/telling your company. In contrast Fulltime Employment Contract which makes it usually impossible to work with another company.
You have to send an invoice to the employer every month.
As it is not considered a safe, fulltime employment it becomes tricky to apply for loans as long as you are working as a contractor.
Note: It isn't uncommon to be initially hired as in an Outsourcing contract position that later turns into a Fulltime Employment Contract. In practice companies intend to keep the person on for a long-term position but do not want to jump to the legal hoops that come with that. As a result it might very well be that the contract is for long-term but you are at the will of the employer.