An syringe assembly (10) of the present invention is composed of an outer cylinder (2) having an outer cylinder body part (21) having a piercing needle fixing part (22) at its distal end and a piercing needle (6) which has a piercing needle tip (61) at its distal end and is fixed to the piercing needle fixing part (22) of the outer cylinder and a seal cap (3) mounted on the outer cylinder (2). The seal cap (3) has a closed distal end part (31), an open proximal end part (32), a cylindrical hollow part (30) capable of accommodating the piercing needle fixing part (22) therein, and a pierceable part (33) into which the piercing needle tip (61) of the piercing needle (6) inserted into the hollow part (30) is pierceable. The seal cap (3) is formed of a rubber composition containing a mixture of butyl rubber and diene-based rubber as its base polymer. The piercing needle tip (61) of the piercing needle (6) is pierced into the pierceable part of the seal cap.
