PURPOSE: To perform high-precision detection by constructing with a high- hardness material only at a portion contacting with a detecting lever of a reference part of thickness and by preventing wear of the reference part of thickness with simple and less expensive materials.

CONSTITUTION: A conveying path 1 is composed of a conveying belt 4 spanned under tension horizontally by being fed to belt-supporting rollers 3 and of a conveying belt 5 spanned in such a way that a portion of which is similar with this conveying belt 4. Facing this conveying path 1 a thickness-reference member 6 is installed. the contacting part of a detecting lever 10 of the thickness-reference member 6 is made of a high-hardness material and by this arrangement wear of the thickness-reference member 6 is prevented.