Structurally new antioxidative metabolite was isolated from mycelial culture of mushroom Stereum hirsutum. Culture broth was subjected to Diaion HP-20 column chromatography, and 70% aqueous MeOH eluent was extracted with EtOAc. EtOAc extract was purified through silica gel and Sephadex LH-20 column chromatographies, and reversed phase $C_{18}$ HPLC. Compound was revealed to be new dihydroxylated derivative of sterin B with molecular formula of $C_{12}H_{16}O_5$(MW 240) by MS and various NMR spectral data analyses, and designated as sterin C. Sterin C showed superoxide radical-scavenging activity with $EC_{50}$ value of 0.31 mM.