To solve the problem that a conventional HST mounting structure, which a HST pump and a HST motor are furnished in a transmission case and connected with by setting up a filter outside of the case through piping, increases cost and causes troubles in building when production and maintenance.

The mission case is divided into front and rear cases, the HST pump (2) and the HST motor are mounted between both the cases, and a port block (6) of which inside is processed from an oil route of the HST is mounted. The filter (7), which connects with the oil route of the HST, is mounted on the side of the port block (6).