PURPOSE: To effectively obtain the subject compound useful as a raw material for medicines, perfumes, agricultural chemicals, etc., by selectively oxidizing only the 4-methyl group of 3,4-xylenol with a microorganism, Pseudomonas putida strain.

CONSTITUTION: This.4-hydroxy-2-methyl benzoic acid useful as a raw material for medicines, perfumes, agricultural chemicals, etc., is effectively obtained by inoculating a medium with a microorganism Pseudomonas putida strain [e.g. Pseudomonas putida KS-0160 strain (FERM P-12879)], vibration-culturing the strain under conditions comprising 30°C, a pH of 6.8 and 180rpm, adding 3,4-xylenol to the culture solution, further vibration-culturing the strain in the culture solution under conditions comprising 30°C, a pH of 6.8, and 180 rpm for 24 hrs, centrifuging the culture solution for removing the cells, adjusting the pH of the obtained supernatant to 2, saturating the adjusted supernatant with sodium chloride, extracting the saturated solution with ethyl acetate, dehydrating and concentrating the extracted solution, and subsequently purifying the obtained crude product, thus selectively oxidizing only the 4-hydroxy group of the 3,4-xylenol.