PURPOSE: To detect the length of paper sheets such as banknotes in the direction at a right angle to the conveying direction thereof accurately by scanning indices which are beforehand disposed on the outside of a measuring region and the end parts of the paper sheets with sensor arrays.

CONSTITUTION: Indices 5, 6 which are beforehand disposed in the positions where the distance between the same is larger than the length in the direction at a right angle to the conveying direction of paper sheets such as banknotes 1 are detected with image sensors 3, 4. Both end parts of the banknotes conveyed with a conveyor belt 2 are detected respectively with the sensors 3, 4. When the detected values of the sensors 3, 4 are arithmetically processed, the length of the banknotes which are long in the direction at a right angle to the conveying direction is detected accurately without providing any sensor array or the like over the entire area for measurement.