PURPOSE: To improve the resolution without use of a lens system and without virtual processing of increasing the number of picture elements by shifting an image sensor in the main scanning direction by a prescribed amt. of shift so as to read data for plural number of times.

CONSTITUTION: An original is read by using an image sensor and read data Pri, Pgi and Pbi are stored in a memory. Then the same original is read at a position at which the image sensor is shifted in the main scanning direction by a prescribed shift quantity and read data Pri', Pgi' and Pbi' are stored in the memory. Furthermore, the same original is read at a position at which the image sensor is shifted in the main scanning direction by a prescribed shift quantity and read data Pri", Pgi" and Pbi" are stored in the memory. When the reading is executed by reading through shifting of two times, since three data read at a different position per one pixel are provided, it is equivalent that the resolution is multiplied by three times.