To integrally form a transmission system without enlarging them by juxtaposing a variable displacement pump composed of a swash plate type plunger pump and a variable displacement motor composed of a swash plate type plunger motor, and installing a pair of communicating passages and required respective valves in a cover.

A variable displacement pump 1 and a variable displacement motor 3 are incorporated into a case 11. A swash plate type plunger pump to maintain a delivery quantity in a zero state at the set range of a tilt angle set in the vicinity of 0 degree, is adopted as the variable displacement pump 1. A swash plate type plunger motor freely variable to high/low two speeds and capable of varying an angle of the inclined face 35 of a swash plate 33 is adopted as the variable displacement motor 3. While, a relief valve, a check valve and a directional control valve are incorporated into a cover 48. Therefore, in spite of adopting the variable displacement motor 3, the case 11 and the cover 48 can be integrally formed without enlarging them.