PURPOSE: To produce a monocarboxylic acid useful as a raw material of surface active agent, lubricant oil, nylon, etc., by culturing a microbial strain belonging to Pseudomonas genus and capable of producing a monocarboxylic acid, in a medium added with a specific aliphatic amine or aliphatic diamine.

CONSTITUTION: A microbial strain belonging to Pseudomonas genus and capable of producing monocarboxylic acid, e.g. Pseudomonas sp. K95 (FERM-P No.7041), is cultured in a medium added with one or more kinds of aliphatic amine of formula R(CH2)nNH2 (R is H or amino; n is 6W18) and/or aliphatic diamine, under aerobic condition, and the produced and accumulated monocarboxylic acid of formula R(CH2)n-1COOH is separated from the medium.