PURPOSE: To obtain the subject compd. at a high yield by changing isobutyreneoxide to R,S-2,2-dimethylcyclopropane carboxylate under the presence of a base by using a phosphonoacetic acid trialkylester, then hydrolyzing the same under presence of the base.

CONSTITUTION: A isobutyreneoxide of formula I is changed to an R,S-2,2- dimethylcyclopropane carboxylate of formula III by a phosphonoacetic acid trialkylester (more particularly phosphonoacetic acid triethylester) of formula II (R1 to R3 are each an alkyl) in the presence of a base. More preferably, the R,S-2,2-dimethylcyclopropane carboxylic acid of formula IV is obtd. by executing the hydrolysis under the presence of the base without isolating the intermediate. The first reaction is recommended to be executed at 100 to 140°C and the ester hydrolysis at 40°C up to the reflux temp. The high yield is attained by this method and the method is simple and cost effective.