PURPOSE: To enable easy measurement of the amt. of the oil coated on a sample with high accuracy by providing an IR gas analyzer, an arithmetic circuit which makes prescribed calculation and multiplies the output signal from the arithmetic circuit by a prescribed coefft.

CONSTITUTION: An arithmetic circuit 11 is so constituted as to calculate the difference between the output signal 9a from an IR gas analyzer 9 when the oil of a sample 2 burns and the concn. of CO2 in the gaseous mixture increases and the output signal 9a from the analyzer 9 when the oil of the sample 2 does not burn and the gaseous mixture is formed only by the gas for supporting combustion and to perform time integration operation of said difference. The output signal 11a from the circuit 11 is then the value corresponding to the overall volume of the gaseous CO2 generated from the oil and said volume depends on the amt. and kind of the oil. The output signal 11a from the circuit 11 is thus made to the value corresponding to the amt. of the oil coated on the sample 2 by setting the coefft. with a multiplier circuit 15 so as to correct the signal 11a from the circuit 11 according to the kind of the oil. The amt. of the oil coated on the sample 2 is thus easily measured by the signal 12a.