A syringe assembly includes an outer cylinder having a cylindrical piercing needle mounting part, a piercing needle fixed the cylindrical piercing needle mounting part and a seal cap. The piercing needle has a piercing needle tip at a distal end. The seal cap has a closed distal end part, an open proximal end part, and a pierceable part into which the piercing needle tip is pierceable. The seal cap is formed of a rubber composition containing a mixture of butyl rubber and diene-based rubber as a base polymer thereof, and the piercing needle tip is in a state of being pierced into the pierceable part of the seal cap.
