This invention relates to estimation of a particulate matter deposit amount in a diesel particulate filter (1) provided in an exhaust passage (7) of a vehicle diesel engine (10). When the vehicle speed exceeds a predetermined vehicle speed, a differential pressure method, in which the particulate matter deposit amount is estimated based on the differential pressure upstream and downstream of the filter (1), is applied, and when the vehicle speed does not exceed the predetermined vehicle speed, an accumulation method, in which the particulate matter deposit amount is estimated by accumulating an increment determined from the load and rotation speed of the diesel engine (10), is applied. When switching from the differential pressure method to the accumulation method, the particulate matter deposit amount determined in the differential pressure method is used as an initial value of the accumulation, and thus the particulate matter deposit amount can be determined accurately.