There is provided a linear cartridge-type motorized dental syringe in which a plunger rod is arranged in alignment with a drive shaft of a motor with a reduction gear (geared motor) serving as a prime mover and advance drive of the plunger rod is attained by a simple mechanism. The linear cartridge-type motorized dental syringe comprising an externally threaded plunger rod (32) located in alignment with an axis of a drive shaft of the geared motor, the plunger rod being movable longitudinally of the casing but not rotatable relative to the casing, a rotating cylindrical body (40) adapted to be rotated by the drive shaft, a nut (57) including a pair of opposite arm-like nut halves (56) having internal threads adapted to engage external threads on the plunger rod, a locking ring (70) located for rotation around a longitudinal axis of the plunger rod and for longitudinal movement between a rearward position wherein the locking ring forces the internal threads on the arm-like nut halves to engage the external threads on the plunger rod, and a forward position wherein the locking ring releases the mutual engagement of the internal and external threads, pusher pins (75) adapted to be pushed by the cartridge holder (22) for moving the locking ring (70) from the forward position to the rearward position when the cartridge holder is inserted in and coupled to cylindrical space (24), and a coil spring (80) for pushing the locking ring (70) back to the forward position when the cartridge holder is not in the cylindrical space, the rotation of the rotating cylinder body (40) resulting in rotative motion of the nut (57) around the plunger rod (32) to linearly move forward the plunger rod (32).
