Abstract: A medical syringe 2 includes a cylindrical syringe body 3 and a plunger 4 fitted in the syringe body 3 for injecting a liquid agent filled in the syringe body 3 from the syringe body 3 into the vascular system of the human body by advancing the plunger 4 relative to the syringe body 3. The plunger 4 comprising a plunger body 5 and an elastic cover 9 fitted over the plunger body 5 from the front. The plunger body 5 is provided with a backplate 62 at a rear end thereof, the backplate 62 having an annular groove 68 forwardly opened and formed in a front surface outer peripheral portion thereof. The portion of the elastic cover 9 to be rearwardly stretched by the advance of the plunger 4 is allowed to ingress into the annular groove 68 for escape.