PURPOSE: To measure the length of merchandise traveling on a conveyer simply, positively, and accurately.

CONSTITUTION: When two sets of optical sensors 4A/48 and 6A/6B are provided and merchandise with a length L=L0+a passes with the spacing L0 between optical axes 4C and 6C which are in parallel with a drive roller 3 as a reference length, the first optical sensors 4A and 4B where the optical axis 4C is shielded first start receiving light after merchandise 1 travels by a since it travels and the leading edge shields the optical axis 6C of the second optical sensors 6A and 6B. A rotary encoder 5 which is mounted to a drive rotar 3 starts counting after the light axis 6C of the second optical sensors 6A and 6B shields light and a rotary encoder 5 measures the length a and the optical sensors 4A, 4B, 6A, and 5B measure the length L0 when counting continuous until the optical sensors 4A and 4B starts receiving light again.