Disclosed are a conjugated diene-based polymer from which a polymer composition excellent in fuel cost-saving properties and elongation at break can be obtained, a polymer composition containing the conjugated diene-based polymer and a reinforcing agent, and a process for producing the conjugated diene-based polymer. There is provided a conjugated diene-based polymer having a conjugated diene unit, a monomer unit based on a compound represented by the following formula (1) and a monomer unit based on a compound represented by the following formula (2). wherein R11 represents a hydrogen atom or a hydrocarbyl group, m is 0 or 1, R12 represents a hydrocarbylene group, and X1, X2 and X3 each independently represent a substituted amino group, or a hydrocarbyl group optionally having a substituent, provided that at least one of X1, X2 and X3 is a substituted amino group. wherein R21 represents a hydrogen atom or a hydrocarbyl group, n is 0 or 1, R22 represents a hydrocarbylene group, and A represents a substituted amino group, or a nitrogen-containing heterocyclic group.