Abstract: The present invention is characterized by including: a first member having a large diameter part and a small diameter part; a second member configured to be connected to the first member; and an element disposed between the first member and the second member, the first member having a recess on an end surface, the recess having a bottom that reaches a boundary position between the large diameter part and the small diameter part or a certain position on the side of the second member relative to the boundary position, the second member having a projection that is configured to fit into the recess with the element disposed between the recess and the projection, and having a screw hole extending to an inside of the projection to allow a screw part of the plunger to be screwed thereinto.

Abstract: Provided is an injection device including an engagement releasing mechanism configured to release engagement between a plunger and an engaging member and to cause the plunger to be located at an end point in a manner that the plunger moves from a start point toward the end point by an urging force of a plunger urging portion and the plunger and the engaging member rotate relative to each other around a central axis.