The present invention offers an operation method of an ozonizer and an ozonizer apparatus to improve ozone gas purity and to achieve long and safety electrolysis operation in such manner that, during normal operation of the ozonizer, ozone gas is generated at the anode in the anode compartment and hydrogen gas is generated at the cathode in the cathode compartment; and only when the ozonizer is stopped and operation is switched to protective current operation during which minute electric current is supplied to protect said anode, oxygen-containing gas is supplied to said cathode compartment after electrolyte and hydrogen gas in said cathode compartment are all drained out, so that said cathode is made function as a gas electrode for oxygen reduction reaction, using said cathode as a reversible electrode with two functions as a gas generation electrode and a gas electrode, thereby during normal operation, ozone is generated efficiently, and during the protective current operation, when safety is a key issue, hydrogen gas is not generated at the cathode and mingling of hydrogen gas into ozone gas generated at the anode is prevented.