Abstract: An oil pump includes a rotation shaft, a pump unit including an inner rotor and an outer rotor, and a casing provided with a pump chamber accommodating the pump unit, wherein at least one of a first suction passage, a first ejection passage, a second suction passage, and a second ejection passage of the casing includes a check valve provided in the oil carrying passage and the check valve includes a ball and a seating portion allowing the ball to be seated thereon and does not use a spring.