This focus detection device uses electric charge accumulation type photoelectric conversion devices. The time period for the next episode of electric charge accumulation by these photoelectric conversion devices is calculated based upon the time period for the current episode of electric charge accumulation, so as to keep the maximum level of an electrical signal for focus detection output from the photoelectric conversion devices less than or equal to a previously determined target level. The accumulation of electric charge by the photoelectric conversion devices is then performed according to this calculated electric charge accumulation time period, and the resultant electrical signal for focus detection is output. A monitor photoelectric conversion device is provided for monitoring the electric charge accumulation state of the photoelectric conversion devices, and this monitor photoelectric conversion device outputs a monitor electrical signal correlated to the average amount of electric charge accumulation of the photoelectric conversion devices. If the electrical signal for focus detection output by the photoelectric conversion devices comes to exceed the maximum input level for its A/D conversion device, then the standard level for the monitor electrical signal is reduced, so as to ensure quick termination of the next episode of electric charge accumulation by the photoelectric conversion devices.