PURPOSE: To provide the higher accuracy and the smaller size by sealing a UV irradiating lamp and a photodetecting part for correction in a hermetic case and utilizing the output ratio of the respective photodetecting parts for correction and measurement when gaseous ozone does not exist.

CONSTITUTION: A microcomputer 3 is started without connecting a quartz glass tube 5 to a vent tube 10 and a source voltage is impressed to the lamp 14, the photodetecting part 5 for correction, the photodetecting part 2 for measurement and an indicator 4 to start these parts. The pressing of the initial ratio setting button of the microcomputer 3 is detected after the internal initial setting is completed and the prescribed time elapses. The ratio of the output signals (initial values) of the photodetecting parts 15, 2 is written into a built-in memory when the above pushing is detected. The sensitivity of the photodetecting parts 15, 2 when there is no absorption of the UV light by the ozone and when the incident light on the photodetecting part 2 does not attenuate. The tube 5 is then connected to the tube 10 and the ozone-contg. air flows. The correction signal calculated by dividing the ratio of the measured values of the photodetecting parts 15, 2 by the initial value ratio is multiplied by the constant stored in a ROM to calculate the concn. of the ozone which is then displayed on an indicator 4. The fluctuation in the source voltage and the fluctuation in the lamp output are compensated in this way, by which the reduction of the size of the device and the measurement with the high accuracy are enabled.