Provided is a method of diagnosing regeneration failure of an exhaust emission control device including filter regenerating means for forcibly heating a particulate filter 13 so as to burn off captured particulates. An exhaust flow rate to the particulate filter 13 and a differential pressure between entry and exit sides of the particulate filter 13 are obtained to calculate a gradient of change in the differential pressure relative to change in the exhaust flow rate. The calculated gradient is compared with a predetermined reference gradient to diagnose the particulate filter 13 as having regeneration failure when the calculated gradient exceeds the reference gradient.