Ozone production facilities comprising oxygen production facilities as a starting gas supply source, an ozonizer, and an ozone analyzer, and adapted to supply the starting gas by the oxygen production facilities, ozonize the starting gas by the ozonizer to produce an ozone-containing gas; the ozone production facilities further including liquid oxygen facilities as another starting gas supply source, and an oxygen flowmeter for monitoring the flow rate of oxygen supplied by the liquid oxygen facilities. A method of operating the ozone production facilities comprises supplying the starting gas from the oxygen production facilities alone to the ozonizer when the consumption of oxygen is within the production capacity of the oxygen production facilities; or mixing pure oxygen gas from the liquid oxygen facilities with the gas from the oxygen production facilities, and supplying the resulting mixed gas as the starting gas to the ozonizer, when the consumption of oxygen exceeds the production capacity of the oxygen production facilities. Thus, the ozone production facilities, even if small in scale, can always supply an ozone-containing gas having an arbitrary ozone concentration, ranging from the maximum to a lower concentration.