PROBLEM TO BE SOLVED: To provide a compound having NaPi2b inhibitory action.SOLUTION: The present invention provides a compound represented by the following formula [I] or a pharmaceutically acceptable salt thereof (ring A1 is a structure represented by formula [II-a], [II-b], or [II-c] ring A2 is phenyl, 3-pyridyl or pyrazole-4-yl X is a single bond or -NH- R3 is a specific substituent).SELECTED DRAWING: NoneCOPYRIGHT: (C)2018,JPO&INPIT【課題】NaPi2b阻害作用を有する化合物の提供。【解決手段】下記式[I]で表される化合物又はその製薬学的に許容される塩。(環A1は式[II-a]、[II-b]、又は[II-c]で表される構造;環A2はフェニル、3-ピリジル又はピラゾール-4-イル;Xは、単結合又は-NH-;R3は、特定の置換基)【選択図】なし