PURPOSE: To easily obtain the title compound as e.g. an intermediate for synthesizing phenols through e.g. the cumene process by oxidizing a secondary or tertiary alkyl group-contg. compound and adding a specific basic compound to the resulting reaction fluid containing the product followed by heating and concentration.

CONSTITUTION: Firstly, a secondary or tertiary alkyl group-contg. compound is oxidized by an oxygen-contg. gas such as air or pure oxygen to produce a secondary or tertiary alkyl hydroperoxide (The conditions for the oxidation is as follows: for the cumene oxidation process, the reaction is carried out at 80-120°C under a pressure of 0-10kg/cm2G for 1-3hr using a base such as NaOH as stabilizer). Thence, a basic compound of rubidium or cesium is added to the reaction fluid followed by heating and concentration, thus obtaining the objective compound. In the heating and concentration, the pH values of the reaction fluid and concentrate are maintained at 3-7.5 (pref. 4-7), respectively. The heating temperature is pref. 100-120°C.