A compound inducing apoptosis in HeLa cells was isolated from Rumex japonicus and identified as musizin-8-O-${eta}$-D-glucoside using basic analyses, including nuclear magnetic resonance spectroscopy, high-performance liquid chromatography, and mass spectrometry. The effect of the compound on inducing apoptosis was elucidated using flow cytometry, Western blot analysis, and confocal immunofluorescence microscopy. The compound isolated from R. japonicus induces apoptosis via the activation of caspaces-3 and -7 in HeLa cells.