[Problem] A main object of the present invention is to provide a novel crystal of 2-{4-[N-(5, 6-diphenylpyrazin-2-yl)-N-isopropylamino]butyloxy}-N-(methylsulfonyl)acetamide (hereinafter referred to as "compound A"). [Means for Resolution] A Form-I crystal of compound A, showing diffraction peaks at 9.4 degrees, 9.8 degrees, 17.2 degrees and 19.4 degrees in the powder X-ray diffraction spectrum thereof. A Form-II crystal of compound A, showing diffraction peaks at 9.0 degrees, 12.9 degrees, 20.7 degrees and 22.6 degrees in the powder X-ray diffraction spectrum thereof. A Form-III crystal of compound A, showing diffraction peaks at 9.3 degrees, 9.7 degrees, 16.8 degrees, 20.6 degrees and 23.5 degrees in the powder X-ray diffraction spectrum thereof.