According to the present invention, provided are dihydropyridazine-3,5-dione derivatives of formula (I), a salt thereof, and solvates of the derivative and of the salt. Also provided are the following that contain any of said compounds as an active ingredient: a drug, a drug composition, a sodium-dependent phosphate transporter inhibitor, and a prophylactic and/or therapeutic agent as well as a prophylactic and/or therapeutic method for hyperphosphatemia, secondary hyperparathyroidism, and chronic renal failure.
