To provide a device for detecting position of a diaphragm by which an F-value shown by an index provided in an iris ring coincides with the F-value of the diaphragm electrically recognized by the output value of a potentiometer by correcting the output value of the potentiometer for detecting the set position of the diaphragm, that is, such a failure that the F-value of the diaphragm when the diaphragm is set to the F-value instructed by an iris command value by servo-control does not coincide with the F-value shown by the index is eliminated.

An iris block 30 includes the diaphragm arranged in a photographic lens and the iris ring for adjusting the position of the diaphragm. The device for detecting the position of the diaphragm has a motor 32 driving the diaphragm through the iris ring and the potentiometer 34 detecting the position of the diaphragm. A CPU 40 reads the output value of the potentiometer 34, recognizes the present F-value of the diaphragm and drives the motor 32, to servo-control the diaphragm, for obtaining the F-value instructed by the iris command value.