PURPOSE: To obtain the titled substance in high yield and safely, by oxidizing a 10-(2-hydroxyphenyl)-decan-1-ol with an aqueous solution a dialkali metallic salt of nitrosodisulfonic acid produced by electrolytic oxidation method as an oxidizing agent.

CONSTITUTION: A compound shown by formula I (R1 and R2 are lower alkyl; n is 0W21; X is H or OH which may be protected; Y is OH which may be protected) is oxidized with an aqueous solution of a dialkali metallic salt of nitrosodisulfonic acid, obtained by electrolytic oxidation of an aqueous solution of a dialkali metallic salt of hydroxylaminedisulfonic acid, as an oxidizing agent to give a compound shown by formula II useful as a drug having immunostimulating action, relaxing action on smooth muscle and activating action on enzyme of cerebral tissue. Use of an expensive oxidizing agent such as silver oxide or an oxidizing agent having possibility of admixture of metal such as FeCl3, etc., is not required.