The purpose of the present study is to examine the antimicrobial activity of a new compound, 3, 4-o-isopropylidene-3, 3', 4, 5'-tetrahydroxy-stilbene (1) chemically derived from 3, 3', 4, 5'-tetrahydroxystilbene (2) which was isolated from the heartwood of Cassia garrettiana CRAIB by the authors, and the antifungal activities of which were previously reported. It has become apparent that the antifungal activities of compound (1) are far stronger than those of compound (2), and that the acute toxicity of compound (1) is lower than that of compound (2). It should be emphasized that compound (1) shows its strong antifungal activity especially against Trichophyton rubrum and Trichophyton mentagrophytes.