It is made possible to determine image characteristics in a reading image while the image of an original is being read. A first line sensor (9R2, 9G2, 9B2) is arranged on a board and reads the original image. A second line sensor (9K1) has a larger number of pixels than the first line sensor and is arranged on the board to read the original image earlier than the first line sensor. An image signal processing characteristic control unit (46) uses an output of the second line sensor as a control signal to control the processing characteristic of the image signal read by the first line sensor.