A syringe comprising: a syringe outer tube including a tubular-shaped outer tube main body configured to be filled with liquid, a reduced diameter section provided at a distal end of the outer tube main body in a projecting manner, and configured to allow the liquid to pass therethrough, the reduced diameter section having an outside diameter that is smaller than a diameter of the outer tube main body, and a ring-shaped lock adapter concentrically disposed at an outer circumferential portion of the reduced diameter section and configured to fix a mating member connected to the reduced diameter section; and a cap including a cap main body mounted on the syringe outer tube and configured to seal the reduced diameter section in a liquid-tight manner when the cap main body is in a mounted state, a ring-shaped mounting section disposed at a proximal end side of the cap main body and mounted on an outer circumferential portion of the lock adapter, and a breaking section that forms a link between the cap main body and the mounting section and that is configured to release the link by breakage, characterized in that the cap main body includes a base portion, a ring portion formed on the proximal end side of the base portion in a projecting manner and a plate-like sealing member disposed inside the base portion and configured to seal a distal end opening of the reduced diameter section in a liquid-tight manner in the mounted state, and wherein, in the mounted state, the ring portion is positioned in a loosely-fitted state between the reduced diameter section and the lock adapter and/or on the outer circumferential side of the lock adapter, the sealing member contacts the distal end opening of the reduced diameter section and configured not to be fitted to a spout formed of the reduced diameter section and the lock adapter, and the ring portion contacts the outer circumferential portion of the reduced diameter section and/or the outer circumferential portion of the lock adapter when an entire portion of the cap main body is tilted to an axis of the syringe outer tube, and wherein the cap main body is not remountable on the syringe outer tube once separated from the mounting section by breakage of the breaking section. a syringe outer tube including a tubular-shaped outer tube main body configured to be filled with liquid, a reduced diameter section provided at a distal end of the outer tube main body in a projecting manner, and configured to allow the liquid to pass therethrough, the reduced diameter section having an outside diameter that is smaller than a diameter of the outer tube main body, and a ring-shaped lock adapter concentrically disposed at an outer circumferential portion of the reduced diameter section and configured to fix a mating member connected to the reduced diameter section; and a cap including a cap main body mounted on the syringe outer tube and configured to seal the reduced diameter section in a liquid-tight manner when the cap main body is in a mounted state, a ring-shaped mounting section disposed at a proximal end side of the cap main body and mounted on an outer circumferential portion of the lock adapter, and a breaking section that forms a link between the cap main body and the mounting section and that is configured to release the link by breakage, characterized in that the cap main body includes a base portion, a ring portion formed on the proximal end side of the base portion in a projecting manner and a plate-like sealing member disposed inside the base portion and configured to seal a distal end opening of the reduced diameter section in a liquid-tight manner in the mounted state, and wherein, in the mounted state, the ring portion is positioned in a loosely-fitted state between the reduced diameter section and the lock adapter and/or on the outer circumferential side of the lock adapter, the sealing member contacts the distal end opening of the reduced diameter section and configured not to be fitted to a spout formed of the reduced diameter section and the lock adapter, and the ring portion contacts the outer circumferential portion of the reduced diameter section and/or the outer circumferential portion of the lock adapter when an entire portion of the cap main body is tilted to an axis of the syringe outer tube, and