An object is to provide a syringe that is capable of preventing a cap from being mounted to a syringe body at an oblique angle. The cap includes an elastic cylindrical body for receiving an injection needle therein and sealing a holding part, and an external cylindrical body for covering the circumference of the elastic cylindrical body and holding the elastic cylindrical body in such a manner as to prevent movement of the elastic cylindrical body. The external cylindrical body includes an opening part having a dimension configured to be larger than the holding part of the syringe body, and a guide part that guides the injection needle toward a certain direction by coming into contact with a part of the syringe body when the injection needle is received inside the elastic cylindrical body.