An object of the present invention is to provide a conjugated diene-based polymer capable of providing a polymer composition which is excellent in a reduction in fuel consumption when silica is incorporated as a filler, a process for producing the conjugated diene-based polymer, and a polymer composition comprising said conjugated diene-based polymers and silica. The present invention provides a conjugated diene-based polymer comprising a conjugated diene-based constitutional unit and a constitutional unit expressed by the following formula (I), wherein said constitutional unit expressed by the formula (I) is between one of the conjugated diene-based constitutional units and another one: (I) (wherein X<1>, X<2> and X<3> represent independently a group expressed by the following formula (II), a hydroxide group, or a hydrocarbon group, and at least one of X<1>, X<2> and X<3> is the group expressed by the formula (II), or a hydroxide group: (II) wherein R<1> and R<2> represent independently a hydrocarbon group having 1 to 6 carbon atoms which can contain a nitrogen atom, an oxygen atom, or a silicon atom, and R<1> and R<2> can be combined to form a ring structure).