Provided is a stabilized solid preparation of 2-{4-[N-(5,6- diphenylpyrazin-2-yl)-N-isopropylamino]butyloxy}-N- (methylsulfonyl)acetamide. A solid preparation containing 2-{4-[N-(5,6-diphenylpyrazin- 2-yl)-N-isopropylamino]butyloxy}-N-(methylsulfonyl) acetamide and D-mannitol having a specific surface area of 1.0 m