A motorized syringe is provided for use with both of two types of dental anesthetic solution-filled cartridges, a plunger rubber plug in the 1.8 ml cartridge being capable of being pushed by a push rod by a length required to push a plunger rubber plug in the 1.0 ml cartridge. The motorized syringe includes a cartridge holder (4) capable of being used in common for both of a 1.0 ml cartridge (3b) and a 1.8 ml cartridge (3a), the push rod (6) having a 1.0 ml cartridge- setting position and a 1.8 ml cartridge-setting position drawn back from the 1.0 ml cartridge-setting position so that the movement of the push rod can end at a common terminal position for both 1.0 ml and 1. 8 ml cartridges, a bottomed sleeve (22) adapted to receive the rear end portion of the push rod therein when the push rod is pushed back from the common terminal position, and to be not interfered with by the rear end face of the push rod when it is in the 1.0 ml cartridge-setting position, the sleeve being pushed rearward with the rear end face of the push rod when the push rod is pushed back to the 1.8 ml cartridge-setting position, and a return spring (23) for moving forward the sleeve together with push rod upon its forward movement.
