To provide an automatic injector or the like capable of detecting an abnormality in a liquid chemical feeding path by confirming the state of a unidirectional valve, and capable of preventing injection of a liquid chemical inappropriate for a patient.

The liquid chemical feeding path 30 has a body 57 with a liquid passage formed inside, and the unidirectional valve 51 having a valve body 51 disposed inside the body 57 to open/close the liquid passage by the movement. A sensor 11 for detecting the position of the valve body 51 is disposed in the outer periphery of the unidirectional valve 51. Piston driving means of the liquid chemical injector is configured to operate based on the result of detection of the sensor 11.