A two-component-mixing prefilled syringe kit, comprising; A syringe barrel in which an injection needle is attached to one end, a plunger is slidably inserted from an opening at the other end, and a solid component or a liquid component constituting an injection solution is filled inside; A container in which an outlet is sealed with a seal member that can be pierced by the injection needle, and a liquid component constituting the injection solution is filled inside; A holding member for holding at least one of the syringe barrel or the container so as to be movable along the barrel axis direction of the syringe barrel with the tip of the injection needle facing the outlet of the container; A needle cap which is formed of a material that can be pierced by the injection needle to cover the injection needle from the tip side, and fixed at base end side to a base portion of the injection needle or an outer surface of the syringe barrel;Wherein, a space-forming body which forms a space facing the surface of the seal member with a space sealed from the outside is arranged so as to be in close contact with the surface of the seal member, and the space-forming body is formed of a material which can be pierced by the injection needle at a portion facing the tip of the injection needle along the barrel axis direction of the syringe barrel; and wherein; The needle cap has a tubular portion (2d5) in which a tubular axis direction is along to the direction toward the seal member and an expected piercing point by the injection needle is located at the inner side, on a front portion facing the seal member, The space-forming body is constituted by the front portion of the needle cap and the tubular portion (2d5) in close contact with the surface of the seal member; characterized in that The needle cap has a flange portion(2d4) being in a direction intersecting with the barrel axis direction of the syringe barrel on the outer side surface of the front portion; and The holding member comprises a mount portion for mounting the syringe barrel, a pushing portion for pushing the flange portion (2d4) against the container when moved to the container side along the barrel axis direction of the syringe barrel, and a lock part for holding a state where the pushing portion pushes the flange portion (2d4) against the container. A syringe barrel in which an injection needle is attached to one end, a plunger is slidably inserted from an opening at the other end, and a solid component or a liquid component constituting an injection solution is filled inside; A container in which an outlet is sealed with a seal member that can be pierced by the injection needle, and a liquid component constituting the injection solution is filled inside; A holding member for holding at least one of the syringe barrel or the container so as to be movable along the barrel axis direction of the syringe barrel with the tip of the injection needle facing the outlet of the container; A needle cap which is formed of a material that can be pierced by the injection needle to cover the injection needle from the tip side, and fixed at base end side to a base portion of the injection needle or an outer surface of the syringe barrel;Wherein, a space-forming body which forms a space facing the surface of the seal member with a space sealed from the outside is arranged so as to be in close contact with the surface of the seal member, and the space-forming body is formed of a material which can be pierced by the injection needle at a portion facing the tip of the injection needle along the barrel axis direction of the syringe barrel; and wherein; The needle cap has a tubular portion (2d5) in which a tubular axis direction is along to the direction toward the seal member and an expected piercing point by the injection needle is located at the inner side, on a front portion facing the seal member, The space-forming body is constituted by the front portion of the needle cap and the tubular portion (2d5) in close contact with the surface of the seal member; characterized in that The needle cap has a flange portion(2d4) being in a direction intersecting with the barrel axis direction of the syringe barrel on the outer side surface of the front portion; and The holding member comprises a mount portion for mounting the syringe barrel, a pushing portion for pushing the flange portion (2d4) against the container when moved to the container side along the barrel axis direction of the syringe barrel, and a lock part for holding a state where the pushing portion pushes the flange portion (2d4) against the container.