PURPOSE: To obtain pisiferic acid or an analog thereof usable as a medicine, by treating a plant body or extract thereof containing the pisiferic acid or analog thereof with a fluid in a supercritical state.

CONSTITUTION: A plant, e.g. Chamaecyparis pisifera Endl. var. plumosa Beissn., Chamaecyparis pisifera Endl., etc., of the family Cupressaceae or an extract thereof is treated with preferably CO2 at 5W10°C higher than 31.1°C critical point thereof under ≤300kg/cm2 pressure to extract pisiferic acid or an analog thereof under supercritical condition. If the plant extract is used particularly as a raw material, the solvent in the extract can be removed to industrially and advantageously concentrate the aimed substance.

USE: An antimicrobial agent.