A control unit inputs a control signal depending on whether reading at a high resolution is selected or reading at a low resolution is selected in an image reading unit. A first delay circuit and a second delay circuit output image data that is captured by a first CCD line sensor and a second CCD line sensor to an adder after retaining the image data for a time in accordance with the input control signal. The adder generates color image data by adding the image data that is captured by the first CCD line sensor to the image data that is captured by the second CCD line sensor.