A measuring apparatus is capable of measuring chlorine dioxide gas concentration by correcting for changes in light emission of an LED due to heating and passage of time, and applying the correction to the optical absorption rate of chlorine dioxide gas at a photodetector. The measuring apparatus includes a main body with a measuring passage surrounded by a pair of transparent bodies, into which specimen gas or fresh air is fed and then discharged, and a comparing passage arranged in the vicinity of the measuring passage, into which no specimen gas is fed, first and second LEDs for emitting ultraviolet light which both have identical properties and are mounted at one end of the measuring passage and at one end of the comparing passage, respectively, and first and second photodetectors for receiving light respectively from the first and second LEDs and detecting a light emission amount thereof.