PROBLEM TO BE SOLVED: To ensure piercing of an injection needle or the like through a seal member as required in spite of a simple structure.SOLUTION: This plug body for a vial includes the seal member (8) covering an upper end opening (6) of the vial (2), and a synthetic resin cap (9) pressing the seal member (8) to the upper face of a collar part (7) around the opening (6). The seal member (8) has a flange (10) placed on the upper face of the collar part (7). The cap (9) includes: an upper wall (15) arranged above the flange (10) a cylinder part (16) hanging from the outer peripheral edge of the upper wall (15) and a locking part (18) arranged inside the cylinder part (16). When switched to a totally plugging attitude, the locking part (18) is locked to the lower face of the collar part (7). A stopping protrusion (27) projected downward is formed at an annular part larger than the inner diameter of the opening (6) and smaller than the outer diameter of the flange (10) out of the lower face of the upper wall (15).
