Natural PGI.sub.2 is a local hormone product in vivo mainly at the endothelium of arterial vessels, and it is an important factor for controlling cell functions in vivo by its strong physiological activities such as an anti-platelet activity and a vasodilating activity. There has been an attempt to use it directly as a drug (P. J. Lewis, J. O. Grandy et al, Clinical Pharmacology of Prostacyclin Raven Press, N.Y., 1981). However, natural PGI.sub.2 has in its molecule a vinyl ether bond which is susceptible to hydrolysis and is readily deactivated under a neutral or acidic condition. Thus, because of its chemical instability, it can not be regarded as a good compound for a drug. Therefore, studies have been made to develop chemically stable synthetic PGI.sub.2 derivatives having physiological activities equal to natural PGI.sub.2. Among them, there is a report on a case wherein a fluorine atom was introduced to various sites ("Journal of Synthetic Organic Chemistry, Jpn", vol 42, 794 (1984), and 7-fluoro-PGI.sub.2 derivatives having fluorine introduced at the 7-position have been reported (Japanese Unexamined Patent Publications No. 171988/1982 and No. 243079/1985). Further, PGI.sub.2 derivatives having a cycloalkyl group introduced to the .omega.-side chain in order to improve the pharmacological effect and stability, have been reported (Japanese Unexamined Patent Publication No. 163365/1984). However, there has been no report on PGI.sub.2 having a cycloalkyl group introduced to the .alpha.-side chain or on derivatives thereof.