PURPOSE: To measure an object transferred on a conveyor optically regardless of the size thereof.

CONSTITUTION: A size measuring instrument 10 and the body of measuring apparatus 20 are disposed along the carrying path of a conveyor 1. The body of measuring apparatus 20 comprises a projecting part 21 and a light receiving part 22 disposed oppositely on the opposite sides of the conveyor 1. The light receiving part 22 comprises a spectroscopic diffraction grating 33, and a charge storage line sensor 34 for measuring the quantity of light in the spectroscopy. A signal from the size measuring instrument 10 is fed through a signal processor/controller 40 to the line sensor 34 and the storage time is regulated according to the size of an object A to be measured.