PURPOSE: To obtain correct calculated results at the time of measuring the thickness of a sheet to be measured with a laser beam by only fetching the measured values of the sheet part of the sheet to a data processing section.

CONSTITUTION: In this instrument, the input of data other than those of the width of a sheet S to be measured is prevented by making constant-value data to be outputted to the outside when the thickness output of a thickness meter 10 is equal to or smaller than a fixed value and, a data processing section 20 to perform data processing without adding the constant-value data upon receiving the thickness information.