A gas concentration measuring device including a light emitter and a light receiver which are disposed so as to be opposed to each other with a hollow tube-like measurement pipe interposed therebetween. The device is configured to measure concentration of target gas passing through the measurement pipe using light applied from the light emitter, transmitted through the inside of the measurement pipe, and received by the light receiver. Purge gas guide pipes through which purge gas is introduced into optical systems of the light emitter and the light receiver are connected to a side wall of the measurement pipe. The measurement pipe includes a gas entrance portion having a tapered shape widening from a gas supply port toward a downstream side thereof.