A stabilized solid preparation containing -{4-[N-(5,6-diphenylpyrazin-2-yl)-N-isopropylamino]butylo xy}-N-(methylsulfonyl)acetamide is provided. A solid preparation containing 2-{4~[N-(5,6~diphenylpyrazin-2-yl)-N-isopropylamino]butylo xy}-N-(methylsulfonyl)acetamide and D-mannitol having a specific surface area of 1.0 m2/g or less. 35