A plunger kit (15, 15A, 15B) comprising a piston to be slidably inserted into a liquid medicine storage chamber of a syringe main body part, and a plunger rod (14, 14A, 14B, 14C, 14D) mountable to the piston, wherein the plunger rod (14, 14A, 14B, 14C, 14D) has a rod main body that extends in an axial direction and transmits drive force to the piston, and a mounting part (32, 32A, 32B, 32C, 32E) that projects from a tip end of the rod main body and is for mounting the plunger rod (14, 14A, 14B, 14C, 14D) to the piston, the mounting part (32, 32A, 32B, 32C, 32E) has a shaft part that projects from a tip end part of the rod main body to a tip end side with a substantially constant diameter, an engagement part (36, 36A, 47, 48) that projects from a tip end part of the shaft part in a radial direction and forms an engagement surface (40, 44a, 44b) formed substantially perpendicular to the axial direction, and a conical part disposed on a tip end side of the engagement part (36, 36A, 47, 48) and reduced in diameter in a tapered manner toward a tip end, a recess is formed on the piston, the recess having an opening part (26, 26a) opened in a base end direction, a narrow part communicating with a tip end side of the opening part (26, 26a) and extending in the axial direction, and a storage part communicating with a tip end side of the narrow part and capable of storing the engagement part (36, 36A, 47, 48) and the conical part, the storage part being formed in a hollow shape, the piston has a first side wall part forming the storage part, and a second side wall part forming the narrow part and thicker than the first side wall part, length of the second side wall part in the axial direction is 10% to 40% of an entire length of the piston in the axial direction, and a cut-out part (42, 42a, 42b, 42D, 45a, 45b) obtained by cutting out a part in a circumferential direction is formed on the engagement part (36, 36A, 47, 48) of the plunger rod (14, 14A, 14B, 14C, 14D); characterized in that the plunger kit (15, 15A, 15B) further comprises a slit structure formed over the conical part, the engagement part (36, 36A, 47, 48), and the shaft part, and wherein the engagement part (36, 36A, 47, 48) is elastically deformable inward. wherein the plunger rod (14, 14A, 14B, 14C, 14D) has a rod main body that extends in an axial direction and transmits drive force to the piston, and a mounting part (32, 32A, 32B, 32C, 32E) that projects from a tip end of the rod main body and is for mounting the plunger rod (14, 14A, 14B, 14C, 14D) to the piston, the mounting part (32, 32A, 32B, 32C, 32E) has a shaft part that projects from a tip end part of the rod main body to a tip end side with a substantially constant diameter, an engagement part (36, 36A, 47, 48) that projects from a tip end part of the shaft part in a radial direction and forms an engagement surface (40, 44a, 44b) formed substantially perpendicular to the axial direction, and a conical part disposed on a tip end side of the engagement part (36, 36A, 47, 48) and reduced in diameter in a tapered manner toward a tip end, a recess is formed on the piston, the recess having an opening part (26, 26a) opened in a base end direction, a narrow part communicating with a tip end side of the opening part (26, 26a) and extending in the axial direction, and a storage part communicating with a tip end side of the narrow part and capable of storing the engagement part (36, 36A, 47, 48) and the conical part, the storage part being formed in a hollow shape, the piston has a first side wall part forming the storage part, and a second side wall part forming the narrow part and thicker than the first side wall part, length of the second side wall part in the axial direction is 10% to 40% of an entire length of the piston in the axial direction, and a cut-out part (42, 42a, 42b, 42D, 45a, 45b) obtained by cutting out a part in a circumferential direction is formed on the engagement part (36, 36A, 47, 48) of the plunger rod (14, 14A, 14B, 14C, 14D); characterized in that the plunger kit (15, 15A, 15B) further comprises a slit structure formed over the conical part, the engagement part (36, 36A, 47, 48), and the shaft part, and wherein the engagement part (36, 36A, 47, 48) is elastically deformable inward.