A contact-type image scanner includes an illumination unit, a control unit and a photo-sensing unit. The illumination unit includes a first, a second and a third light source for respectively emitting different color light beams on an original surface. The control unit is in communication with the illumination unit for controlling on durations of the light sources. In response to a first start pulse and a second start pulse, the first light source and the second light source are turned on for a first on duration and a second on duration respectively, wherein the second light source is turned on immediately after the first light source is turned off or after the first light source is turned off for a delaying time. In response to a third start pulse, the first, second and third light sources are simultaneously turned on for a first minor on duration, a second minor on duration and a third minor on duration, respectively. The photo-sensing unit obtains a first image data value during the first on duration, obtains a second image data value during a second on duration and obtains a third image data during the first, second and third minor on durations.