PURPOSE: To obtain an optically active carboxylic acid by a one-stage reaction using a simple separation means by biochemically oxidizing an isoalkyl compound with a specific microorganism.

CONSTITUTION: An isoalkyl compound of formula I (R1 is an alkyl, an aromatic ring, a heterocyclic group or a sulfur-containing alkyl; (m) and (n) are independently 0, 1) is oxidized with a microorganism belonging to the genus Rhodococcus or Mycobacterium and capable of converting the compound of formula I to a carboxylic acid. The optically active carboxylic acid of formula II or formula III (C* is carbon atom imparted with optical activity) is easily separated from the reaction medium by extraction, etc.