To ensure prescribed pressure distribution even when a seal member reaches a grooves' end, by forming dovetail-type grooves at the tip of a ring- shaped gear, and inserting movable seal member into the grooves radially.

A pinion 5 having outer gear teeth which are fixed onto a driving shaft 4 and a ring-shaped gear 6 having inner gear teeth are connected each other deflectively, and they are engaged. A housing 1 includes an intake connection part 7 which communicates with peripheral region of a pump chamber during the pinion 5 rotates in the arrow X direction, and has an exhaust connection part 10 in its opposite region. By entering pressure medium into the pump chamber, that is tooth gaps in the pinion 5 and the gear 6 through a hole which penetrates the gear 6 radially, and inserting seal member 30 into addenda of the gear 6, engagement 12 of the pinion 5 and the gear 6 is sealed.