A drug injection device according to the present invention is provided with: a main body case (2); a drug syringe attachment part (3) provided inside the main body case (2); a piston (5) which is provided in such a way as to be movable with respect to a drug syringe (4) that is detachably fitted to the drug syringe attachment part (3); a drive mechanism (6) for driving the piston (5); a controller (7) which is electrically connected to the drive mechanism (6); and an acceleration sensor (8) which is connected to the controller (7). When the drug syringe (4) is in a manual mixing mode, the controller (7) is switched to the following step (air bleed mode) if the angle of inclination of the main body case (2) detected by the acceleration sensor (8) is greater than a predetermined value.
