A light irradiating device provided with line light sources is located in association with an image recording medium. The line light sources are located in parallel and at approximately identical pitches with respect to a scanning direction. A slit array plate having slits is located between the image recording medium and the light irradiating device. Each of the slits is located at a position corresponding to the position of one of the line light sources. The light irradiating device is controlled such that line light beams are radiated out one after another with different timing from the line light sources.