This syringe assembly 10 comprises: a sheath 2 provided with a sheath main body 21 having a puncture needle fixing part 22 at the distal end thereof and a puncture needle 6 having a puncturing needle tip 61 at the distal end thereof the puncture needle 6 being fixed to the puncture needle fixing part of the sheath and a seal cap 3 mounted on the sheath 2. The seal cap 3 is provided with a blocked distal end part 31 an open proximal end part 32 a cylindrical hollow part 30 capable of accommodating the puncture needle fixing part 22 and a penetrable part 33 which can be penetrated by the puncturing needle tip of the puncture needle 6 inserted into the hollow part 30. The seal cap is formed from a rubber composition having a mixture of butyl rubber and diene based rubber as a base polymer thereof and the puncturing needle tip 61 of the puncture needle 6 penetrates into the penetrable part of the seal cap.
