A syringe assembly comprising: a barrel assembly including: a cylindrical barrel body having an opening at a base end, a barrel tip portion having an annular head and being provided at an end of the barrel body, and a puncture needle having a needle hole and being held in the barrel tip portion so that a tip of the puncture needle projects from the barrel tip portion; and an elastic cap comprising: an open end at one end of the elastic cap, a bottom wall at the other end of the elastic cap, aside wall extending from the bottom wall toward the open end portion, a cylindrical receiving hole formed by the bottom wall and the side wall, the receiving hole configured to receive at least part of the barrel tip portion and the puncture needle, wherein the receiving hole includes: a first inner diameter portion having a smaller diameter than an outer diameter of the annular head; a stopper portion provided at an end of the first inner diameter portion; and a second inner diameter portion that is formed from the stopper portion to the bottom wall, the second inner diameter portion having a smaller diameter than the first inner diameter portion, wherein a minimum diameter of the second inner diameter portion is 2 to 10 times the outer diameter of the puncture needle, and wherein the elastic cap is detachably mounted to the barrel tip portion of the barrel at least part of the barrel tip portion is inserted into the receiving hole, the first inner diameter portion is in close contact with an outer peripheral surface of the annular head such that an airtight seal is formed between the first inner diameter portion and the barrel tip portion, the stopper portion abuts against an end face of the annular head such that the stopper portion defines a final insertion position of the barrel tip portion, and the tip of the puncture needle is inserted into the bottom wall such that the bottom wall seals the tip of the puncture needle, characterized in that the elastic cap is formed of a single elastic member. a barrel assembly including: a cylindrical barrel body having an opening at a base end, a barrel tip portion having an annular head and being provided at an end of the barrel body, and a puncture needle having a needle hole and being held in the barrel tip portion so that a tip of the puncture needle projects from the barrel tip portion; and an elastic cap comprising: an open end at one end of the elastic cap, a bottom wall at the other end of the elastic cap, aside wall extending from the bottom wall toward the open end portion, a cylindrical receiving hole formed by the bottom wall and the side wall, the receiving hole configured to receive at least part of the barrel tip portion and the puncture needle, a cylindrical barrel body having an opening at a base end, a barrel tip portion having an annular head and being provided at an end of the barrel body, and a puncture needle having a needle hole and being held in the barrel tip portion so that a tip of the puncture needle projects from the barrel tip portion; and an elastic cap comprising: an open end at one end of the elastic cap, a bottom wall at the other end of the elastic cap, aside wall extending from the bottom wall toward the open end portion, a cylindrical receiving hole formed by the bottom wall and the side wall, the receiving hole configured to receive at least part of the barrel tip portion and the puncture needle, an open end at one end of the elastic cap, a bottom wall at the other end of the elastic cap, aside wall extending from the bottom wall toward the open end portion, a cylindrical receiving hole formed by the bottom wall and the side wall, the receiving hole configured to receive at least part of the barrel tip portion and the puncture needle, wherein the receiving hole includes: a first inner diameter portion having a smaller diameter than an outer diameter of the annular head; a stopper portion provided at an end of the first inner diameter portion; and a second inner diameter portion that is formed from the stopper portion to the bottom wall, the second inner diameter portion having a smaller diameter than the first inner diameter portion, a first inner diameter portion having a smaller diameter than an outer diameter of the annular head; a stopper portion provided at an end of the first inner diameter portion; and a second inner diameter portion that is formed from the stopper portion to the bottom wall, the second inner diameter portion having a smaller diameter than the first inner diameter portion, wherein a minimum diameter of the second inner diameter portion is 2 to 10 times the outer diameter of the puncture needle, and wherein the elastic cap is detachably mounted to the barrel tip portion of the barrel at least part of the barrel tip portion is inserted into the receiving hole, the first inner diameter portion is in close contact with an outer peripheral surface of the annular head such that an airtight seal is formed between the first inner diameter portion and the barrel tip portion, the stopper portion abuts against an end face of the annular head such that the stopper portion defines a final insertion position of the barrel tip portion, and the tip of the puncture needle is inserted into the bottom wall such that the bottom wall seals the tip of the puncture needle, characterized in that the elastic cap is formed of a single elastic member.