An image reader that reads the image on transmission-type originals such as negative reversal films for use on digital photoprinters, for example, and which has an inexpensive and simple construction and yet is capable of efficient and rapid image reading operations. The time of reading the light passing through a transmission-type original by a main scanning CCD assembly is varied in accordance with the image data on the original. Alternatively, a prescanning unit and main scanning unit are provided as independent entities that use a prescanning CCD assembly and a main scanning CCD assembly, respectively.