When flash photography is performed, an exposure time at a time of a preliminary light emission of a flash is set as short as possible so that the appropriate amount of light required for a main light emission of the flash can be accurately computed. Before the main light emission of the flash, an exposure operation prior to preliminary light emission and an exposure operation at the time of preliminary light emission are performed. After each of these exposures, the pixel signals of individual pixels are intermittently read out at predetermined pixel intervals. The brightness levels of images captured in the exposure operation prior to preliminary light emission and the exposure operation at the time of preliminary light emission are individually detected based on the signals. The amount of light required for the main light emission of the flash is computed based on a difference between the brightness levels.