A medical syringe comprising: a tubular syringe barrel; a gasket fitted in the syringe barrel to seal the syringe barrel; and a plunger having a distal end portion kept in threaded engagement with a threaded hole of the gasket, and reciprocally movable in the syringe barrel, wherein the gasket is a laminated gasket for use in a medical syringe, comprising: a main body made of an elastic material; and a film provided on a surface of the main body; the gasket having a circumferential surface portion to be kept in contact with an inner peripheral surface of a syringe barrel of the syringe, a liquid contact surface connected to one of opposite edges of the circumferential surface portion, a bottom surface provided on an other edge side of the circumferential surface portion, and a threaded hole extending from a center of the bottom surface into the main body and adapted to be in threaded engagement with a distal end portion of a plunger of the syringe; wherein the threaded hole has an internal thread provided on an inner peripheral surface thereof as extending from a mouth thereof toward an innermost portion thereof and including a plurality of periodic thread portions; characterized in, provided that at least an innermost one of the periodic thread portions of the internal thread adjacent to the innermost portion of the threaded hole has a diameter F1, that the other periodic thread portions of the internal thread each have a diameter F2 and that at least a distal one of periodic thread portions of an external thread provided on the distal end portion of the plunger has a diameter D1, a relationship F2 > D1 > F1 is satisfied, wherein for improving the gas-tightness when a thread of the plunger is in threaded engagement with the innermost thread portion of the threaded hole of the gasket, the smaller thread diameter portion of the gasket body is pressed outward and, therefore, the lamination film is more strongly pressed against the inner wall of the syringe barrel. a tubular syringe barrel; a gasket fitted in the syringe barrel to seal the syringe barrel; and a plunger having a distal end portion kept in threaded engagement with a threaded hole of the gasket, and reciprocally movable in the syringe barrel, wherein the gasket is a laminated gasket for use in a medical syringe, comprising: a main body made of an elastic material; and a film provided on a surface of the main body; the gasket having a circumferential surface portion to be kept in contact with an inner peripheral surface of a syringe barrel of the syringe, a liquid contact surface connected to one of opposite edges of the circumferential surface portion, a bottom surface provided on an other edge side of the circumferential surface portion, and a threaded hole extending from a center of the bottom surface into the main body and adapted to be in threaded engagement with a distal end portion of a plunger of the syringe; wherein the threaded hole has an internal thread provided on an inner peripheral surface thereof as extending from a mouth thereof toward an innermost portion thereof and including a plurality of periodic thread portions; characterized in, provided that at least an innermost one of the periodic thread portions of the internal thread adjacent to the innermost portion of the threaded hole has a diameter F1, that the other periodic thread portions of the internal thread each have a diameter F2 and that at least a distal one of periodic thread portions of an external thread provided on the distal end portion of the plunger has a diameter D1, a relationship F2 > D1 > F1 is satisfied, wherein for improving the gas-tightness when a thread of the plunger is in threaded engagement with the innermost thread portion of the threaded hole of the gasket, the smaller thread diameter portion of the gasket body is pressed outward and, therefore, the lamination film is more strongly pressed against the inner wall of the syringe barrel. a main body made of an elastic material; and a film provided on a surface of the main body; the gasket having a circumferential surface portion to be kept in contact with an inner peripheral surface of a syringe barrel of the syringe, a liquid contact surface connected to one of opposite edges of the circumferential surface portion, a bottom surface provided on an other edge side of the circumferential surface portion, and a threaded hole extending from a center of the bottom surface into the main body and adapted to be in threaded engagement with a distal end portion of a plunger of the syringe; wherein the threaded hole has an internal thread provided on an inner peripheral surface thereof as extending from a mouth thereof toward an innermost portion thereof and including a plurality of periodic thread portions; characterized in, provided that at least an innermost one of the periodic thread portions of the internal thread adjacent to the innermost portion of the threaded hole has a diameter F1, that the other periodic thread portions of the internal thread each have a diameter F2 and that at least a distal one of periodic thread portions of an external thread provided on the distal end portion of the plunger has a diameter D1, a relationship F2 > D1 > F1 is satisfied, wherein for improving the gas-tightness when a thread of the plunger is in threaded engagement with the innermost thread portion of the threaded hole of the gasket, the smaller thread diameter portion of the gasket body is pressed outward and, therefore, the lamination film is more strongly pressed against the inner wall of the syringe barrel.