To securely recognize a work in photographing and recognizing the work even when the brightness of an operation position where the work is set varies.

In order to recognize the shape and position of a work through a camera 1, a learning mode is entered, and while a master work is photographed, the aperture of an iris diaphragm 2 provided to the camera 1 is varied to obtain the optimum aperture corresponding to illuminance; while the aperture is set at the optimum value, a parameter value is varied to obtain and store the optimum parameter value when the feature quantity of the master work is closest to a target value in a memory 5. Then a recognition mode is entered, and while the iris diaphragm 2 is adjusted to the optimum aperture stored in the memory 5 corresponding to the illuminance, the work is recognized according to the optimum parameter value.