An assembly (10) of the present invention for a syringe includes a piercing needle-attached barrel (2) having a barrel body and a hollow piercing needle (6) fixed to the barrel body and a seal cap (3) for sealing a piercing needle tip (61). The seal cap has a hollow part (30) and a pierceable part (33) into which the piercing needle tip (61) inserted into the hollow part (30) can be pierced. The seal cap is water vapor permeable and placed in a state in which the piercing needle tip has been pierced into the pierceable part. At least the pierceable part (33) of the seal cap (3) is formed of an elastic material containing silicone rubber as a main material thereof, silicone particles, and a lubricant.
