PURPOSE: To maintain the reaction system thermally stable using an alkyl metal complex as the polymerization catalyst by polymerizing an anionically polymerizable monomer in the coexistence of a complex compound obtained by reacting a specified organometallic compound and a first complexing agent and a second complexing agent.

CONSTITUTION: The object polymer represented by the formula is obtained by polymerizing at least one anionically polymerizable monomer selected from among a linear conjugated diene monomer, a vinylaromatic monomer, a polar monomer, ethylene, an α-olefin monomer, etc., in the presence of a complex compound as the main polymerization catalyst formed by reacting an organometallic compound containing a group IA metal in the periodic table with a first complexing agent and a second complexing agent (which may be the same or different from the first complexing agent). In the formula, A to D are each a monomer unit constituting the main chain of the polymer; a to d are the weight percentages of A to D based on the total weight of A to D, respectively; A is a unit derived from a linear conjugated diene monomer; B is a unit derived from a vinylaromatic monomer; C is unit derived from a polar monomer; and D is a unit derived from ethylene or an α-olefin monomer.