To provide a liquid phase polymerization method for manufacturing a polylefin elastomer using a crosslinked metallocene as a catalyst composition.

In this method for manufacturing an elastomer by a liquid phase polymerization of ethylene, other α-olefins and, if desired, a diene monomer, the elastomer is obtained by polymerizing the monomers under a liquid phase polymerization condition by employing a metallocene procatalyst, which is at least one of compounds represented by the general formulae (I): (CP1R1m)R3n(Cp2R2p)MXq and/or (II): (Cp1R1m)R3nYrMXs [wherein Cp1 and Cp2 are the same or different and are a cyclopentadienyl ring: R1 are R2 are each independently a halogen or a hydrocarbyl, halocarbyl, hydrocarbyl-substituted organic metalloid or a halocarbyl-substituted organic metalloid group containing carbon atoms up to abut 20; Y is a heteroatom-containing ligand coordinated with M; N is a transition metal having a valence of 3-6; and each X is independently a halogen, or the like] in combination with a cocatalyst.