Five oxanthrone C-glycosides, namely rumejaposide A-E, and an epoxynaphthoquinol, together with eight known compounds, 2,6-dihydroxy benzoic acid, 4-hydroxy benzoic acid, epicatechin, 4-hydroxy-3-methoxy benzoic acid, 2,6-dimethoxy-4-hydroxyl benzoic acid, rutin, emodin and 2-acetyl-1,8-dihydroxy-3-methyl-6-methoxynaphthalene, were isolated from the roots of Rumex japonicus. The structures of the oxanthrone C-glycosides were elucidated by application of spectroscopic methods as (10R)10-C-beta-glucopyranosyl-1,8,10-trihydroxy-2-carboxyl-3-methyl-9(10H)-anthracenone, (10S)10-C-beta-glucopyranosyl-1,8,10-trihydroxy-2-carboxyl-3-methyl-9(10H)-anthracenone, (10R)10-C-beta-glucopyranosyl-1,6,8,10-tetrahydroxy-2-carboxyl-3-methyl-9(10H)-anthracenone, (10R)10-C-beta-glucopyranosyl-1,6,8,10-tetrahydroxy-3-hydroxymethyl-9(10H)-anthracenone, and (10R)10-C-beta-glucopyranosyl-1,6,8,10-tetrahydroxy-3-methyl-9(10H)-anthracenone. Absolute configurations for each compound were deduced by analyses of CD spectra and comparison with those known similar compounds. The structure of epoxynaphthoquinol was elucidated by spectroscopic methods as 3-acetyl-2-methyl-1,4,5-trihydroxy-2,3-epoxynaphthoquinol, and its relative configuration was determined by a 2D-ROESY experiment.