An oil pump includes a drive gear, which rotates with a drive shaft, a driven gear, which meshes with the drive gear and is supported rotatable on a driven shaft, and a casing having a pump chamber, which accommodates the drive gear and the driven gear. The casing of the gear pump is provided with an intake port and a discharge port, which are in fluid communication with the pump chamber, so that oil is sucked through the intake port and discharged through the discharge port. A gear holder is provided in the casing such that the gear holder supports the driven gear rotatable and holds both sides of the driven gear while the gear holder itself is being supported axially movable by the driven shaft. The gear holder holding the driven gear moves axially by receiving a biasing force and also a pressing force acting against the biasing force.