To minimize unneeded image scan and to eliminate a troublesome operation.

In the case a selection switch of a scan start mode is pressed down erroneously when 2nd and 3rd modes are set (S2 and S3) and also a read start instruction is made from a host computer (Y in S4), a 1st mode is never set (S1) by prohibiting the 1st mode setting. Then, unneeded image scan can be minimized, and a scan start operation needed by an operator can be performed.