A plunger pump (20, 22) comprising: a cylinder (1, 15, 16) having a cylinder chamber, and a first through hole and a second through hole that open from an inner circumferential surface toward an outer circumferential surface of the cylinder chamber; and a plunger (11, 11', 11") inserted in the cylinder chamber and configured to make a reciprocating motion with respect to the cylinder chamber, wherein an inner circumferential portion of the cylinder (1, 15, 16) has an inner circumferential surface, and a helical first groove portion (3, 31, 32) on the inner circumferential surface, the helical first groove portion (3, 31, 32) being configured to communicate with at least one of the first through hole and the second through hole, the inner circumferential surface of the cylinder (1, 16) has a second groove portion (4, 17) communicating with the first groove portion (3, 32) and extending over an entire circumference in a circumferential direction of the cylinder (1, 16), or an outer circumferential surface of the plunger has a second groove portion configured to communicate with the first groove portion (3, 31, 32) and extending over an entire circumference in a circumferential direction of the cylinder (1, 15, 16), and an inner circumferential surface of the second groove portion comprises a first side surface and a second side surface opposite to each other, and a bottom surface connecting the first side surface and the second side surface to each other, and a maximum grain size of crystal grains on the first side surface and the second side surface is smaller than a maximum grain size of crystal grains on the bottom surface. a cylinder (1, 15, 16) having a cylinder chamber, and a first through hole and a second through hole that open from an inner circumferential surface toward an outer circumferential surface of the cylinder chamber; and a plunger (11, 11', 11") inserted in the cylinder chamber and configured to make a reciprocating motion with respect to the cylinder chamber, wherein an inner circumferential portion of the cylinder (1, 15, 16) has an inner circumferential surface, and a helical first groove portion (3, 31, 32) on the inner circumferential surface, the helical first groove portion (3, 31, 32) being configured to communicate with at least one of the first through hole and the second through hole, the inner circumferential surface of the cylinder (1, 16) has a second groove portion (4, 17) communicating with the first groove portion (3, 32) and extending over an entire circumference in a circumferential direction of the cylinder (1, 16), or an outer circumferential surface of the plunger has a second groove portion configured to communicate with the first groove portion (3, 31, 32) and extending over an entire circumference in a circumferential direction of the cylinder (1, 15, 16), and an inner circumferential surface of the second groove portion comprises a first side surface and a second side surface opposite to each other, and a bottom surface connecting the first side surface and the second side surface to each other, and a maximum grain size of crystal grains on the first side surface and the second side surface is smaller than a maximum grain size of crystal grains on the bottom surface.