A method capable of stably performing continuous production of a propylene polymer with high productivity while reducing generation of agglomerates is described. In the method, a monomer(s) containing propylene is/are (co)polymerized in a presence of an olefin polymerization catalyst with a polymerization system containing two or more gas phase polymerization reactors or a polymerization system containing a liquid phase polymerization reactor(s) and a gas phase polymerization reactor(s) such that that the total number of liquid phase polymerization reactor(s) and gas phase polymerization reactor(s) is three or more. In at least one gas phase polymerization reactor, an average retention time τG [hour] in the gas phase polymerization, an average particle diameter Dpi [μm] of fed powder, and a total amount Co [wt %] of an ethylene-derived structural unit and C4-C12 α-olefin-derived structural units in a polymer in discharged powder are in a predetermined relationship.