PURPOSE: To economically obtain the title compound useful as an intermediate for production of corticoid because of remarkably reduced amount of an used isomerizing agent by isomerizing an acyloxy-methylene-al-steroid with a specific isomerizing agent.

CONSTITUTION: A steroid having partial structure expressed by formula I (R is hydrocarbon residue; wave line represents that each of E type and Z type may be used as steric configuration of acyloxy and formyl bonded to a carbon at 20 site) is isomerized using a non-nucleophilic strong base (e.g. tetrabutylammonium fluoride) as an isomerizing agent to provide the aimed compound having a partial structure expressed by formula II. Furthermore, the isomerization reaction is carried out normally in a polar organic solvent (preferably DMF, etc.) and reaction temperature is preferably 30-100°C and reaction time is normally preferably 1-12hr.