A gas concentration measurement device which utilizes a TDLAS measurement method, and in which the phase-sensitive detection can be performed by digital processing using an integer-arithmetic device, is provided. In the gas concentration measurement device according to the present invention, AC components corresponding to integer multiples of a modulation frequency f contained in an input signal are removed by taking a moving average of data obtained from an output signal of a multiplier 62 for a period of time corresponding to one cycle of the modulation frequency f . As a result, a DC component in the output signal of a digital filter 63 relatively increases, making it easier to extract the DC component by a digital low-pass filter 64, so that a sufficiently accurate phase-sensitive detection can be made even if a digital processing based on integer arithmetic is used.