Removal of front end blockage of a diesel particulate filter

A particulate matter deposit amount accumulated in a diesel participate filter (4) is calculated as a first particulate matter deposit amount from a pressure loss of the filter (4) (S33). The particulate matter deposit amount accumulated in the filter (4) is calculated as a second particulate matter deposit amount from another parameter (S34). It is determined whether or not there is front end blockage of the filter (4) based on a comparison between the first particulate matter deposit amount and second particulate matter deposit amount (S36, S43). When there is a front end blockage of the filter (4), the regeneration device is controlled to perform regeneration of the filter (4) for a longer period than the normal regeneration period of the filter (4) (S22).