Image data is inputted from a data processing device 23 into storage means 24 formed on a light-emitting element (yellow) line head 28 so that light emitting elements in one line 28a expose pixels on an image carrier in response to output signal from a shift resistor 24a. As the image carrier is moved in the direction of X to bring said pixels to reach the position corresponding to light emitting elements in the next line 28b, the image data is transmitted to a shift resistor 24b so that the shift resistor 24b outputs the image data to light emitting elements in the line 28b, whereby the pixels are exposed to light again. The movement of the image carrier and the transmission of the image data to the respective shift resistors are sequentially conducted, thereby conducting multiple exposure of each same pixel.