An image server includes: a focus position control unit for determining a focus position based on a maximum value of focus evaluation detected by a picture signal level detection unit; and a focus lens driver for moving the focus lens in predetermined control volumes based on an instruction from the focus position control means; characterized in that, in case the exposure time is changed to a longer cycle than a predetermined cycle, the focus position control unit determines a large control volume corresponding to the change in exposure time and moves the focus lens with the control volume to detect the maximum value of focus evaluation and performs focus control.