To make catalysts active earlier by homogenizing the active states of the catalysts carried into an assembly line of a factory.

An assembly method for an exhaust emission control device which is provided for an internal combustion engine and has an exhaust pipe and the catalyst for purifying exhaust gas includes a catalyst assembly step and an exhaust pipe connection step, the exhaust pipe having a catalyst holding part, and an expansion part formed on the upstream side of the catalyst holding part while expanding in the radial direction. The assembly method further includes an active state measuring step of measuring the active state of the catalyst, an exhaust temperature measuring step of measuring the exhaust temperature of the internal combustion engine, and an expansion part measuring step of measuring the axial dimension of the expansion part, and also includes a selecting step of collating the active state of the catalyst, the exhaust temperature of the internal combustion engine, and the dimension of the expansion part of the exhaust pipe with one another, selecting the exhaust pipe to be combined with the catalyst whose active state approximates a reference value, and delivering it to the catalyst assembly step.