PURPOSE: To remove dirt not intended by the user by storing dirt data on a contact glass pane and eliminating the stored dirt data from read image data so as to obtain a correct image.

CONSTITUTION: An image read section 2 reads optically a read range on a contact glass pane with no original mounted to obtain dirt data of the contact glass pane and the data are stored in a memory 16. Read image data of an original including the dirt data of the contact glass are stored in a memory 17. A dirt data eliminating section 18 subtracts data in the memory 16 from the data in the memory 17 to obtain a correction image from which the dirt data are eliminated. An analysis section 19 analyzes the dirt data in the memory 16 and informs it to the user that the dirt or flaw of the contact glass is spread over a prescribed amount.