A controller (70) controls regeneration of an exhaust gas filter (52) which traps particulate matter contained in exhaust gas to prevent the particulate matter from being discharged into the atmosphere. When regeneration of the exhaust gas filter (52) is underway and the engine (10) is decelerating, the controller (70) determines whether or not a catalyst (40) is active (S82) and whether the engine (10) is decelerating gently or rapidly (S83). When the engine (10) is decelerating gently, the controller (70) performs a post-injection following a main fuel injection (S824), and when the engine (10) is decelerating rapidly, the controller (70) performs the main fuel injection but stops the post-injection (S834). In so doing, the temperature of the filter does not rise excessively, and the filter can be prevented from melting.