The chemical dosing device

The present invention is to downsize the entire device. The present invention, initial as contact in the initial state, at a position the piston 21 is pulled than pulled back position, and a portion of the frustoconical which is formed in the distal end portion 21A is a packing 27 over the circumferential direction is arranged at a position, by the retracted position of the piston 21 before the chemical solution is injected into the drug solution reservoir 6, volume collapse of the packing 27 when the piston 21 is in the initial position is reduced, packing 27 of the piston 21 body it is possible to reduce the contact resistance than when in contact with the part 21B, without a large force in order to start the piston receives a starting resistance at the start of use, it is possible to miniaturize the entire device.