A plunger kit comprising a piston to be slidably inserted into a liquid medicine storage chamber of a syringe main body part , and a plunger rod mountable to the piston , wherein the plunger rod has a rod main body that extends in an axial direction and transmits drive force to the piston , and a mounting part that projects from a tip end of the rod main body and is for mounting the plunger rod to the piston , the mounting part 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 that projects from a tip end part of the shaft part in a radial direction and forms an engagement surface formed substantially perpendicular to the axial direction, and a conical part disposed on a tip end side of the engagement part 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 opened in a base end direction, a narrow part communicating with a tip end side of the opening part 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 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 obtained by cutting out a part in a circumferential direction is formed on the engagement part of the plunger rod ; characterized in that the plunger kit further comprises a slit structure formed over the conical part , the engagement part , and the shaft part , and wherein the engagement part is elastically deformable inward. wherein the plunger rod has a rod main body that extends in an axial direction and transmits drive force to the piston , and a mounting part that projects from a tip end of the rod main body and is for mounting the plunger rod to the piston , the mounting part 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 that projects from a tip end part of the shaft part in a radial direction and forms an engagement surface formed substantially perpendicular to the axial direction, and a conical part disposed on a tip end side of the engagement part 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 opened in a base end direction, a narrow part communicating with a tip end side of the opening part 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 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 obtained by cutting out a part in a circumferential direction is formed on the engagement part of the plunger rod ; characterized in that the plunger kit further comprises a slit structure formed over the conical part , the engagement part , and the shaft part , and wherein the engagement part is elastically deformable inward. <EOS>