The main purpose of the present invention is to provide a novel crystal of 2-{4-[N-(5,6-diphenylpyrazine-2-yl)-N-isopropylamino]butyloxy} acetic acid (hereinafter "compound B"). A type I crystal of compound B with which, in a powder X-ray diffraction spectrum obtained by using CuKα radiation (λ=1.54 Å), the diffraction angle (2θ) shows peaks at the diffraction angles 6.4°, 8.1°, 9.5°, 10.9°, 13.2°, 15.7°, 17.0°, 19.5°, 20.3°, 21.0°, and 22.8°. A type II crystal of compound B with which, in a powder X-ray diffraction spectrum obtained by using CuKα radiation (λ=1.54 Å), the diffraction angle (2θ) shows peaks at the diffraction angles 9.6°, 11.4°, 11.7°, 16.3°, 17.5°, 18.5°, 18.7°, 19.9°, 20.1°, 21.0°, and 24.6°.
