A leading flank (15G) of each thread (15A) of an external screw (15) and a groove flank (14G) of a corresponding groove (14A) of an internal screw (14) have mutually-different angles (³=100°,´=90°) with respect to a push direction and pull direction of a piston, respectively. When the piston is slid in the push direction in a syringe barrel, a hard contact part (15C) and soft contact part (15D) occur on the leading flank. Increased thread engagement strength can hence be provided between the external screw and the internal screw without substantially increasing screwing resistance. A following flank (15H) of the thread and an opposite groove flank (14H) of the groove are also constructed similarly.
