Provided is an imaging device with which a phase-difference amount can be efficiently calculated, and the speed of phase difference autofocus can be increased. An imaging element (5) is provided with imaging pixels (51), and phase-difference detection pixels (51R, 51L). When a video-imaging start instruction is present, a defocus-amount calculation unit (19) determines, in accordance with information related to F value, focal distance, and focus position, a shift amount for two output signal groups during correlation calculation. The defocus-amount calculation unit (19) performs correlation calculation by offsetting the two output signal groups by respective unit amounts within a range of the determined shift amount, and calculates the defocus amount from a result of the correlation calculation. If the distance to a main object subsequently changes, the preset shift amount is changed to a greater value.