A pump device 1 includes a relief valve 42 having a ball 45 and a valve seat 44. The valve seat 44 has a ring-shaped seat surface 52 having a wide portion 52a and a narrow portion 52b on which the ball 45 is seated. Upon valve opening, the ball 45 is pressed in a valve opening direction by a pressing force F1 according to a pressure P of working fluid while being pressed against the wide portion 52a. The ball 45 rolls on the seat surface 52 upon the valve opening while continuing or keeping contact with the wide portion 52a. At this time, the ball 45 is stably held with the ball 45 contacting the wide portion 52a of the seat surface 52. Noise of the relief valve 42 caused by vibration of the ball 45 is therefore suppressed.