Disclosed is a sustained release preparation which releases a poorly soluble medicinal agent in a pH-independent manner. Also disclosed is a sustained release preparation which is capable of controlling the Cmax of a medicinal agent to an adequate amount and is thus capable of maintaining the level of the medicinal agent in the blood to a level at which medicinal effects can be expected for a long period of time. Specifically disclosed is a sustained release preparation which is characterized by containing a pharmaceutically acceptable salt of 4-bromo-6-[3-(4-chlorophenyl)propoxy]-5-(3-pyridylmethylamino)-3(2H)-pyridazinone, and hypromellose. An organic acid is blended in the sustained release preparation in an amount of less than 1% by mass.