A diesel engine (1) comprises an exhaust passage (3), and a DPM filter (4) provided on the exhaust passage (3), which traps diesel particulate matter (DPM) contained in the exhaust gas such that the diesel particulate matter accumulates therein. An engine controller (11) starts regeneration processing of the DPM filter (4) by raising the exhaust gas temperature when a regeneration timing of the DPM filter (4) is reached, sets a target exhaust gas temperature during regeneration processing to ensure that the temperature of the DPM filter (4) does not exceed an upper temperature limit of the DPM filter (4), even when the temperature of the DPM filter (4) is raised due to the engine (1) entering an idling condition during the regeneration processing, and controls the exhaust gas temperature to the target exhaust gas temperature.