The purpose of the present invention is to provide: a methyllactam ring compound having an SGLT1 inhibitory activity and being useful as a medicine or a pharmacologically acceptable salt of the compound; a medicinal composition containing the compound or salt; and medicinal uses of the compound or salt and the medicinal composition. Provided are: the compound of formula [I] or a pharmacologically acceptable salt thereof; a medicinal composition containing the compound or salt; and medicinal uses of the compound or salt and the medicinal composition.