To reduce an influence of a detection error caused by a time lag up to actually performing imaging after an imaging trigger signal is generated.

A template pattern is taken out of first image data, and a moving distance of an object is found by searching a large area of a correlation with the template pattern in second image data. A shift quantity of the object during the time lag up to performing the imaging after generating a trigger signal for acquiring the image data by an image sensor, is found, and a correction is made when finding the moving distance by using this shift quantity.