An assembly (10) of the present invention for a syringe is composed of a barrel (2), made of cyclic polyolefin, which has a barrel body (21) having a distal end part (22) and a piercing needle (6) which has a piercing needle tip (61) at its distal end and is fixed to a piercing needle fixing portion of the barrel and a seal cap (3) mounted on the barrel (2). The seal cap (3) is formed of a thermoplastic elastomer composition containing as its main component a mixture of a styrene-based thermoplastic elastomer and a softener, for hydrocarbon-based rubber, which has a kinematic viscosity of 1 to 5,000mm 2 s -1 at 100 degrees C and a naphthenic carbon ratio (%CN) of not more than 20% when said naphthenic carbon ratio is measured by ring analysis.
