PURPOSE: To smoothly transfer fluid by making an internal gear and an external gear carry out a first engagement and a second engagement at symmetrical sites with respect to a center line each at a contact ratio of one (1) or more, and forming capacity variable spaces between both engaging parts.

CONSTITUTION: An oil pump is formed by an internal gear 1 and an external gear 2, and the external gear 2 is brought into contact with the internal gear 1 in the section about from the point A to the point B and at a contact ratio of one (1) or more. The gears are shiftedly brought into contact with each other at their addendum curved lines at a contact ratio of one (1) or more from the point C to the point E. And, capacity spaces SaWSd are formed in between both gears. Thereby, the outlet and inlet ports for fluid can be easily designed, enabling smooth conveyance of fluid.