NEW MATERIAL:A sesquiterpene expressed by formula I (R is =C=S, formula II or formula III).

USE: An antimicrobial agent.

PREPARATION: For example, a kind of Demospongiae collected in a sea area of Okinawa is homogenized by grinding and subjected to extraction treatment using a proper solvent and isolated by purifying with a column chromatography to afford the compound expressed by formula I (R is =C=S). The compound expressed by formula I (R is =C=S) is subjected to reduction treatment under proper conditions to give the compound expressed by formula I (R is formula II). Then the compound expressed by formula I (R is formula II) is reacted with p-bromobenzoylchloride to provide the compound expressed by formula I (R is formula III).