To provide an opening degree detection device for an automatically operated valve which is easily assembled into an automatically operated valve, makes it easy to ensure detection accuracy at the time of assembling the opening degree detection device, and is easily applicable to an existing automatically operated valve. The opening degree detection device includes: a base plate 4 which is detachably mounted on an automatically operated valve 2; a displacement sensor 5; and a target 6 having an inclined detection surface 6a which is detected by the displacement sensor 5. The target 6 is fixed to the base plate 4. The displacement sensor 5 is supported on a valve stem 14 by way of a sensor support member 21, and the sensor support member 21 is supported on the base plate 4 by way of a guide means.