PURPOSE: To obtain the subject compound with a low alkene conversion rate and a high selectivity of alkylene oxide by feeding an alkene, an oxygen- containing gas, and fire retarding agent into the reaction zone so that they are allowed to react with each other under specific conditions.

CONSTITUTION: An alkene such as ethylene, propylene, an oxygen containing gas (e.g. air, etc.), and a fire retardant (e.g. alkene, etc.), into the reaction zone, allowing the alkene to react with an enzyme-containing gas under conditions of a low alkene conversion rate and a high selectivity, thus obtaining a mixture of alkylene oxide and effluent gas. The obtained mixture is cooled and all the alkylene oxide is taken out, the cooled mixture is devided into the first stream containing unreacted alkene and the second stream containing the remaining unreacted effluent gas, subsequently the first stream is sent into the reaction zone and the second stream is removed, thus producing alkylene oxides from corresponding alkenes.