PURPOSE: To provide the non-contact measuring device capable of enhancing the measuring accuracy of a work by correcting the difference of reflectivity between the respective mirrors of a polygon mirror.

CONSTITUTION: The offset voltage corresponding to the reflectivities of the respective mirrors of a polygon mirror 18 outputted by an offset voltage forming means constituted of a peak hold circuit 34 are subtracted (offset) from the voltage signal outputted from the measuring part 10 of a work 28 by an operational amplifier 38. This offset voltage signal is converted to a binary signal through zero cross comparison by a comparator 40. By this constitution, the voltage signals different in level outputted from the measuring part 10 can be converted to binary signals of the same level and, therefore, the difference of reflectivity between the respective mirrors of the polygon mirror can be corrected.