We sometimes use Limited Partnerships for Investment to establish venture capital, though we have other forms such as the joint stock company, which is often used for the said purpose.
Although the translation in the following site is not updated and people need to check amendments, it might be useful for your reference.
Limited Partnership Act for Investment:
Enforcement Order of the Limited Partnership Act for Investment
You may find a model contract was provided by Ministry of Economy, Trade and Industry, at the following link:
(list of Venture Capitals and Accelerators)
The following site provides a list of venture capitals in Japan.
For your reference, the Accelerators are as follows.