Five anthraquinones, emodin (1), ${omega}$-hydroxyemodin (2), chrysophanol-8-O-${eta}$-D-glucoside (3), emodin-8-O-${eta}$-D-glucoside (4), and physcion-8-O-${eta}$-D-glucoside (5), and five flavonoids, kaempferol-3-O-${eta}$-D-glucoside (6), quercetin (7), quercitrin (8), isoquercitrin (9), and (+)-catechin (10), were isolated from the EtOAc-soluble extract of the fruits of Rumex japonicus. The structures of 1-10 were identified by spectroscopic methods including NMR studies. This is the first report on the isolation of compounds 3-5 from this plant. The isolates were subjected to in vitro bioassays to evaluate their inhibitory activities on the rat lens aldose reductase (RLAR), among which two anthraquinones (1 and 4), and five flavonols (5-9) showed significant activities on RLAR.