A syringe mold with a coolant circulation outward and homeward positioned close to the cavity space and a sufficient coolant circulating flow for cooling a core cylinder is provided.Slide cores (5,5) are put in the position where the slide cores (5,5) protrude from the circumference of the core cylinder (2) by moving the slide core operating jig (4) forward in the axial direction of the core cylinder (2) while cooling the core cylinder (2) by the coolant. Then, a two medical agent mixing type syringe having a communicating groove on the inner circumference for mixing two medical agents is injection-molded. A molding failure due to insufficient cooling of the core cylinder (2) can be prevented by positioning the coolant outward and homeward close to the cavity space and making the cross sectional area of the coolant outward and homeward enough large because the coolant outward and homeward is partitioned by the circumference of the jig container space in the core cylinder (2).
