To simplify and facilitate molding by eliminating necessity to separately provide any gasket or slip by molding the outer peripheral surface of the top end part of a plunger, which can be slid while being tightly fitted with the inner peripheral surface of a barrel and the barrel, into high-accuracy true cylinder.

Concerning a plastic syringe, a main body 1 of barrel having a true cylindrical inner surface injection-molded by any suitable plastic material such as polyethylene terephthalate, for example, has a fitting nozzle 1a for a needle cannula at its top end and a flange 1d at the other end. Besides, a main body 2 of plunger formed from the same plastic material is made into true cylinder so that a top end part 2a can be slid while being tightly fitted into a barrel 1, and an intermediate part 2b between the top end part and a flange 2c at the other end is shaped into cylinder having a diameter a little smaller than that of the top end part 2a. Then, these main body 1 of barrel having the true cylindrical top end part 1a and the main body 2 of plunger having the true cylindrical top end part 2a at least can be comparatively easily molded.