The present invention relates to an elastic cap (18) and a syringe assembly (10) therewith. The elastic cap (18) according to the invention includes a first inner diameter portion (44) having a smaller diameter than an outer diameter (c) of an annular head (22) of a barrel tip portion (16) ； a stopper portion (48) provided at an end of the first inner diameter portion (44) ； a second inner diameter portion (42) that is formed from the stopper portion (48) to a bottom and has a smaller diameter than the first inner diameter portion (44)； a bottom wall (36) provided at an end of the second inner diameter portion (42)； and a receiving hole (20), where a minimum diameter (e) is 2 to 10 times the outer diameter of the puncture needle (12), the stopperportion (48) abuts against an end face (22A) of the annular head (22) to thereby define a final insertion position of the barrel tip portion (16), and a tip of the puncture needle (12) is inserted into the bottom wall (36) to thereby seal the tip of the needle hole (32).
