PURPOSE: To instantaneously detect position displacement of a paper form in the direction orthogonal to the conveying direction of the paper form so as to prevent its jam or the like by judging the paper form displaced to either right or left when both paper form detecting sensors on the left and right sides detect and or do not detect the paper form in the passing of the paper form.

CONSTITUTION: Paper form detecting sensors SW1, SW2 for detecting existence of a passing paper form are arranged interposing a passing route, in its both sides respectively, where at least either a right or left end edge of the paper form passes, as viewed in the paper form conveying direction. The paper form is judged displaced either to right or left when both the paper form detecting sensors SW1, SW2 on both the right and left sides detect and or do not detect the paper form in its passing. By this constitution, position displacement of the conveyed paper form in the direction orthogonal to its conveying direction can be instantaneously detected by a simple device even when picture processing speed becomes higher.