According to the present invention provided are a dihydropyridazine- 3 ,5- dione derivative , 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.
