A modified diene polymer is obtained by decomposing a diene polymer having a carbon-carbon double bond in a main chain by oxidative cleavage of the carbon-carbon double bond to reduce a molecular weight, changing acido-basic properties of a system containing polymer fragments obtained such that the system is changed into a basic system when the system is acidic and the system is changed into an acidic system when the system is basic to combine the polymer fragments, thereby obtaining an intermediate polymer having an altered structure, and epoxidizing a carbon-carbon double bond moiety of a main chain of the intermediate polymer.