There is provided a injector in which a backstop function is realized by a collar part at a position situated away from a gasket in the direction of the side of the proximal end and adequate sterilization can be carried out, with a plunger rod (in which the collar part is formed) assembled, while aiming to secure airtightness. (A backstop function is realized by means of a collar part at a position situated away from the gasket in the direction of the side of the proximal end, thereby providing a injector capable of being adequately sterilized with a plunger rod (in which the collar part is formed) assembled, while aiming to secure airtightness.) A gasket (3) is inserted from a proximal end opening (22) of a barrel (2). Next, a plunger rod (4) is inserted as to establish connection of its distal end and the gasket (3). Then, a finger grip (5) is mounted to a flange (23). A disc part (45) is formed at a position situated away, in the direction of the side of the proximal end, from the gasket, thereby making it possible for the disc part (45) to abut and stop against an aperture edge (521) of the finger grip. There is formed in an outer peripheral edge part of the disc part (45) a communication opening part (451) having a concave shape. At the time of sterilization with hydrogen peroxide gas, the hydrogen peroxide gas which has flowed into an open space (26) from the proximal end opening is permitted to circulate in a semi-hermetically closed space (25) by way of the communication opening part.
