A method for producing a modified conjugated diene polymer includes a step (A) carrying out a primary modification reaction that reacts a heterocumulene compound having two or more specific functional groups with a conjugated diene polymer having a vinyl content of below 10% and a cis-1,4-bond content of 75% or more and possessing an active end to obtain a primary-modified conjugated diene polymer, and a step (B) carrying out a secondary modification reaction that reacts an active hydrogen-containing compound that includes one or more functional groups selected from the group consisting of an amino group, an imino group, a mercapto group, and a hydroxyl group with the primary-modified conjugated diene polymer to obtain a secondary-modified conjugated diene polymer. The method can produce a modified conjugated diene polymer that exhibits low heat build-up (low fuel consumption) and excellent wear resistance.