The method for producing a modified conjugated diene based (co)polymer, according to the present invention comprises (a) obtaining a primarily modified polymer by carrying out a primary modification reaction wherein at least one member selected from the group consisting of an alkoxysilane compound and a silanol compound is introduced into the active alkali metal terminal of a conjugated diene based (co)polymer having an active alkali metal terminal, and (b) obtaining a secondarily modified polymer (a modified conjugated diene based (co)polymer) by subjecting the above-obtained primarily modified polymer to reactions comprising (b1) and (b2). (b1): a reaction with a condensation accelerator comprising a metal element. (b2): a reaction with at least one member selected from the group consisting of inorganic acids and metal halides.