A flash apparatus to perform preliminary flashing, detect an average brightness of surroundings in synchronization with a periodic signal subsequent to the preliminary flashing, and set main flashing time according to the detected average brightness, is controlled by controlling the flash apparatus to start preliminary flashing at a point earlier than a first point at which either a rising edge or a falling edge of a first pulse of the periodic signal is generated, detecting a first average brightness of surroundings at the first point, controlling the flash apparatus to terminate the preliminary flashing at a point later than the first point, detecting a second average brightness of surroundings at a second point at which either a rising edge or a falling edge of a second pulse subsequent to the first pulse of the periodic signal is generated, setting the main flashing time according to the detected first and second average brightness, and controlling the flash apparatus to perform main flashing according to the set main flashing time.