PURPOSE: To obtain a monitoring method and device which can properly judge deterioration and life of an ozone decomposition catalyst at an inexpensive facility cost.

CONSTITUTION: This device is provided with temperature sensors 7 and 8 which are provided at entrance and exit sides, an operator 11 which obtains an increase temperature difference (Tb-Ta) of a delivered ozone according to a catalyst reaction from an output signal of each temperature sensor, an alarm setting equipment 5, a timer 12 for setting judgment time, and an alarm 6 for an ozone decomposition catalyst 3 which is loaded into an ozone decomposition device 2. It is judged that the ozone decomposition catalyst deteriorated under conditions where an increase temperature difference value which is obtained by the operator 11 when the ozone decomposition device 2 operates does not exceed a temperature difference setting value which is determined by the alarm setting equipment 5 within a setting time which is determined by the timer 12. Then, an alarm equipment 6 is operated and an exchange timing of the ozone decomposition catalyst is reported to an outside.