In an injector provided with an injector body a syringe section and a piston a plunger has a rod section which in a mounted state in which the syringe section is mounted to the injector body extends from the base end side end surface of the plunger toward the piston. The piston has on the front end side thereof a housing hole having an opening into which the rod section of the plunger can be inserted and the housing hole is formed so that the base end side end of the rod section is housed therein without being in contact with the piston. In the mounted state in which the rod section is housed within the housing hole the front end side end surface of the piston is in contact with the base end side end surface of the plunger enabling the plunger to slide toward the injection opening side and the plunger in a filling chamber is positioned at a predetermined position with reference to the position of the piston disposed in a through hole the predetermined position being the position where the amount of a substance to be injected which is within the filling chamber is a predetermined amount. Consequently the injector can be handled easily and the filling of a substance to be injected such as an injection solution can be accurately performed.
