A technique is provided that enables reducer concentrations in exhaust gas flowing into an exhaust purification device on the upstream side and an exhaust purification device on the downstream side provided to an exhaust pipe in series to be controlled separately with a simple configuration. Included are the two exhaust purification devices provided to an exhaust passage in series, a bypass passage that bypasses the exhaust purification device on the upstream side, an exhaust control valve provided to the bypass passage, and reducer supply means provided on the upstream side of a branch portion. A reducer is supplied intermittently from the reducer supply means to periodically change the reducer concentration in the exhaust gas, and the exhaust control valve is opened or closed periodically at a predetermined timing with respect to the change in the reducer concentration. Accordingly, the supplied reducer is distributed to be introduced to the exhaust purification device on the upstream side or to be introduced to the exhaust purification device on the downstream side via the bypass passage.