chlorine gas together with calcium oxide the flow rate of oxygen gas can be suitably selected depending on the size of the inner diameter of the gas inlet tube when the gas inlet tube employed is 6 mm in inner diameter the flow rate of oxygen gas should preferably be within the range of 30 ml min to 75 ml min incidentally it has been discovered by the present inventors that in order to enable the fused salt of calcium chloride to excellently react with oxygen the calcium chloride is required to be heated to 800 u00b0 c or more on this occasion in order to prevent the calcium oxide thus produced from being fused the heating temperature should preferably be limited to less than the melting point of the calcium oxide as an alternative embodiment the inlet gas switching four way cock 2 may be omitted and calcium chloride inside the quartz reaction tube 1 may be heated while constantly introducing air therein from the gas inlet tube 6 when the heating temperature is raised beyond the melting point of calcium chloride a fused salt thereof is generated and when the heating temperature is raised beyond 800 u00b0 c the oxygen in the air is allowed to react with the fused salt of calcium chloride to produce.