Provided is a liquid injection device capable of accurately detecting whether a vial is mounted on the mounting portion with an inexpensive configuration. A sensor has a light-emitting part and a light-receiving part and is configured to emit light from the light-emitting part to pass through at least a part of a cylindrical cap attached to the vial to be mounted on a mounting portion and detect an amount of light received by the light-receiving part. An optical axis of the light from the light-emitting part to the light-receiving part is shifted or inclined in a direction that increases a distance that the light passes through the cap relative to a position where the optical axis of the light passes through the center axis within a plane perpendicular to the center axis of the cap.