Provided are a syringe cap, a syringe assembly, and a prefilled syringe capable of sufficiently sterilizing a puncture needle and the like and capable of maintaining the amount of a drug solution as the amount at the time of filling. The syringe cap (10) is detachably attached to the outer cylinder (14) so as to seal the needle hole (26) of the puncture needle (18) held by the cylinder tip (16) of the outer cylinder (14). The The syringe cap (10) is composed of a first member (30) made of an elastic body with a Young's modulus of 1 to 10 MPa into which a puncture needle (18) is inserted, and an elastic hollow body with a Young's modulus of 1 to 10 MPa, It has the 2nd member (32) which forms closed space between a 1st member (30) and an outer cylinder (14). The first member (30) is made of a material having a lower water vapor permeability than the second member (32).穿刺針等を十分に滅菌することができ、且つ薬液の量を充填時の量として維持することができるシリンジキャップ、シリンジ組立体及びプレフィルドシリンジを提供する。シリンジキャップ（１０）は、外筒（１４）の筒先部（１６）に保持された穿刺針（１８）の針孔（２６）を封止するべく、外筒（１４）に着脱自在に装着される。このシリンジキャップ（１０）は、穿刺針（１８）が刺し込まれ、ヤング率が１～１０ＭＰａの弾性体からなる第１部材（３０）と、ヤング率が１～１０ＭＰａの弾性中空体からなり、第１部材（３０）と外筒（１４）との間に閉空間を形成する第２部材（３２）とを有する。第１部材（３０）は、第２部材（３２）に比して水蒸気透過度が小さい材料から形成される。
