The present invention provides an injection device that includes: a syringe 2 including a barrel 21 including a cylindrical barrel body 21a and a nozzle 21b provided at a front end of the barrel body so as to be communicated with the barrel body, a gasket 23 slidable in the barrel, and a plunger 24 connected with the gasket and capable of being placed into and removed from the barrel through a rear end opening of the barrel a finger flange 25 provided in the vicinity of the rear end portion of the barrel body and molded integrally with or separately from the barrel body a holder 3 having an inner diameter that allows the barrel to be inserted in a bore of the holder, attached to the barrel, and including a spiral protrusion 31a formed on the inner surface of a portion of the holder disposed around the nozzle and an engagement structure capable of keeping a state in which the holder is attached to the barrel and releasing the attachment.
