A stereo camera is provided with two imaging units. A CPU detects a position of a CCD provided in each of the imaging units. Based on detection results, the CPU checks whether the CCD has reached an edge of a corresponding correctable area for each of the imaging units. The CPU starts to move the CCDs to their respective reference positions when one of the CCDs has reached the edge of the corresponding correctable area. The CPU controls actuations of CCD shifters such that the CCDs reach their respective reference positions simultaneously.