An injection system has: a gasket having a plurality of engaging claws, wherein each of the engaging claws includes an inner surface S1 which defines a hole H having an inlet with an enlarged diameter and an outer surface S2 inclined in a direction away from a perpendicular line P passing through a center of the hole H, and the engaging claws are to be displaced between a widened position and a narrowed position; a rain to be inserted into the hole H so as to engage with the engaging claws; a cylinder, into which the gasket is to be inserted, contacting the outer surfaces S2; and an injection device configured to move the ram forward and injecting a chemical solution in the cylinder. An annular groove which serves as a starting point of deformation of the engaging claws is formed on the gasket.