Females of Demotina fasciculata (Coleoptera: Chrysomelidae) were found to prefer to feed on galls induced by Andricus moriokae (Hymenoptera: Cynipidae) rather than on leaves of its host plant, Quercus serrata (Fagaceae). This is the first record of cecidophagy by adult chrysomelid beetles. Demotina fasciculata did not infest healthy galls induced by another unidentified cynipid species on the same host trees, but did feed on galls inhabited by an inquiline species Synergus quercicola (Hymenoptera: Cynipidae), presumably because such galls remained on the host trees longer than healthy galls. Galls of A. moriokae were infested more severely than cynipid galls inhabited by the inquiline. Therefore, higher density and thicker gall wall in A. moriokae galls seem to make them more suitable targets for D. fasciculata to attack. Larval chambers of A. moriokae galls were stripped by the infestation of gall walls and readily dropped to the ground, resulting in 100% death of cynipid larvae due to desiccation, while 62.5% of pupae survived when they had developed to the late stadium before the fall of larval chambers.