Disclosed is a dental-caries detecting solution prepared by dissolving a dye capable of staining a carious region of a tooth to provide enhanced visibility of the carious region, in a water-miscible organic solvent, which is capable of distinctively staining only a first decalcified layer, or an outer layer of a softened dentine, to be removed, absolutely without staining a second layer, or an inner layer of the softened dentine, which should not be removed. In the dental-caries detecting solution, the water-miscible organic solvent contains a compound having a carbon number of 11 or more and including one or more hydroxyl groups. The water-miscible organic solvent is adjusted such that the dental-caries detecting solution has a viscosity of 300 to 1500 Cp.