PURPOSE: To simplify a control system, to save energy consumption, and to improve durability of an ozonizer and to perform effective sterilization with ozone of a space in a cabinet of an automatic vending machine or a water reservoir by controlling an ozonizer installed to an automatic vending machine, etc., to operate repeatedly and intermittently with a fixed output.

CONSTITUTION: Concentration of ozone necessary for the sterilization of a space having a fixed volume to be sterilized, such as a space inside of a cabinet of an automatic vending machine, water reservoir, etc., and mean decomposition velocity of ozone under environmental conditions of the space, are determined experimentally previously. Thus, optimum operation cycles and suspending cycles of an ozonizer 2 are determined basing on the results of the above described data. After setting the cycles with a timer 10 provided to an operation control part which operates an ozonizer 2 interposed to a power circuit connected to between an electrode 2a and 2c, the ozonizer 2 is operated to ozonize air fed by a fan 9. Ozone is diffused to attain uniform ozone concn. Thus, sterilization with ozone is performed by repeating operation and suspension periodically for every set time.