The method for producing a modified conjugated diene based (co)polymer, according to the present invention comprises a step of reacting a conjugated diene based (co)polymer with a metal halide compound to obtain a modified conjugated diene based (co)polymer, the conjugated diene based (co)polymer having a weight-average molecular weight of 150,000 to 2,000,000 and being obtained by bonding, to a polymer having at least a conjugated diene unit, an alkoxysilyl group and an optionally protected primary amino group. The method can satisfactorily produce a conjugated diene based (co)polymer which has a high Mooney viscosity, excellent shape stability and good processability.