A new norlignan, styraxlignolide A (1), and two new terpenes, styraxosides A (2) and B (3), were isolated from the MeOH-soluble fraction of Styrax japonica Sieb. et Zucc. (Styracaceae) stem bark, together with two known compounds, egonol (4) and masutakeside I (5). The new compounds were determined as 5-(3″-hydroxypropyl)-7-methoxy-2-(3′,4′-dimethoxyphenyl)-benzofuran 3″-O-[β-D-xylopyranoside-(1→6)-β-D-glucopyranoside] (1), 3β,7β-dihydroxy-4α,4β,8β,10β,14α-pentamethyl-5α-gon-16-en-2-one 3-O-[β-D-glucopyranoside-(1→2)-β-D-glucopyranoside] (2), and 3β,17β-dihydroxy-28-norolean-12-en-16-one 3-O-[α-L-rhamopyranoside-(1→2)-β-D-glucuronopyranoside] (3) by spectroscopic means including 2D-NMR. The five compounds were tested in vitro for anti-complement activity against the complement system. Compounds 1, 3, 4, and 5 displayed inhibitory activity in the anti-complement assay, with IC50 values of 123, 65, 33, and 166 μM, respectively. Compound 1a and camellenodiol (3a), obtained from acid hydrolysis of 1 and 3, respectively, did not affect the hemolytic activity of human serum against sensitized erythrocytes. This shows that a sugar seems to play a role of enhancing significantly anti-complement activity.