To provide a groove position detecting device wherein groove position is detected with precision even if discrimination of groove is hard because of low groove contrast, brightness is not stable except for groove part due to rough on surface condition of a part, no even lighting condition is obtained within the same part, or average brightness among parts is varying.

The device is provided with an imaging means 23 for imaging an object 11 whose groove position is detected. The differential value of brightness data between a pixel of interest and a not-adjoining proximity pixel in the image data on the surface of the object imaged by the imaging means 23 is obtained, which is compared with a threshold, and if the differential value of the brightness data is judged to be greater than the threshold, the interesting pixel is taken as at a groove position.