A process for the production of a 3,3-dimethyl-2-formylcyclopropanecarboxylic acid derivative of formula (2): ##STR00001## wherein R is hydrogen, substituted or unsubstituted alkyl, substituted or unsubstituted aryl, or substituted or unsubstituted aralkyl, which process comprises reacting a 3,3-dimethyl-2-(2-methyl-1-propenyl)cyclopropanecarboxylic acid compound of formula (1): ##STR00002## wherein R is as defined above, with a periodic acid compound in the presence of a ruthenium compound.