Abstract: A process for the preparation of an aqueous hinokitiol solution, characterized by mixing hinokitiol or hinokitiol containing oil with water, boiling the obtained liquid mixture by heating, rapidly cooling the resulting liquid mixture, blowing oxygen into the liquid mixture under stirring, and storing the obtained liquid mixture at a temperature of 0.5 to 5.degree. C. The process can give an aqueous solution of the water-insoluble essential oil having a concentration of up to about 40%. This aqueous solution of the oil is easily handleable by virtue of its liquidness, and does not contain any organic solvent harmful to the human body, thus being highly safe. Further, the solution has antibacterial and/or microbicidal effect.