PURPOSE: To make the structure of a detecting part simple and compart and to perform the measurement of a boundary by one scanning, by selecting signals from photoelectric conversion elements which are located in a completely bright and in a completely dark region, when the boudary between the bright and dark regions crosses the photoelectric conversion element at the center.

CONSTITUTION: A reference photoelectric conversion element 1e serves as a detecting reference of the position of a boundary. Reference photoelectric conversion elements 1aW1d are also provided, in such a way that one element is located in a completely bright region and the other one is located in a completely dark region, when the boundary between the bright and dark regions crosses the element 1e. A detecting part 1 is constituted by the elements 1e and the reference element group wherein the elements 1aW1d are arranged on the same plane as the element 1e. Circuits 3e, 6c, 6d, 7, and 8 are provided so as to select the output signals out of the elements, which are located in the completely bright region and in the completely dark regions, based on the output signals of the elements 1aW1d. When the output of the element 1e agrees with the reference level based on the difference signal between the two selected output signals, a coincident signal is obtained, and a sound generating device 10 and an indication meter 11 are driven.