PURPOSE: To prevent leak by cutting-off the communication to the outside when the capacity of the pressure chamber of a trochoid type hydraulic apparatus becomes min., and by reducing the gap between the teeth between the capacity increase side and the reduction side by the pressure in the pressure chamber.

CONSTITUTION: The position of each communication hole 30 formed on a side plate which communicates to a pressure chamber formed between a rotor 19 constituted of a trochoid tooth form and a stator 22 engeged with the rotor 19 is set closer to the center side of the side plate in comparison with the conventional case. As for the pressure chamber A' in which the capacity is made min., the communication hole 30 is shut-off by the teeth of the rotor 19, and the internal pressure of the pressure chamber A' is increased, and the rotor 19 is pressed onto the between pressure chambers A and A' side on the capacity increase side and the reduction side, and the gap P is reduced.