Image pickup is executed at an image pickup timing based on reference pulses by using, as a trigger, workpiece pulses which are output at an interval synchronized with a shape portion (projecting or recess portion) which is periodically and repetitively formed on a workpiece being rotated, and the image pickup is executed at the image pickup timing based on the reference pulses PA every time only reference pulses PA whose number corresponds to a pulse number between workpiece pulses are output from the image pickup timing concerned.