Abstract: Gaseous-phase polymerization of an .alpha.-olefin with a catalyst and without a liquid dispersant is carried out under the following conditions:1. A solid phase comprising said catalyst and particulate olefin polymer is caused principally by mechanical agitation to undergo circulation in the up-and-down directions within a reaction chamber of substantially vertical-cylinder type, a reaction bed being formed by said circulating solid phase;2. The ratio of the diameter to height of said reaction bed is of the order of from 1 : 0.5 to 1 : 3;3. The starting-material .alpha.-olefin is supplied in liquid state into the reaction chamber from a point above the reaction bed and vaporized within the reaction chamber; and4. Heat of polymerization reaction is removed principally by the latent heat of vaporization absorbed by said vaporization of the liquid .alpha.-olefin.