A diesel particulate filter (“DPF”) system is provided with temperature sensors at an entrance side and an exit side of a diesel oxidation catalyst, an SV ratio determining device that determines an exhaust gas SV ratio by measuring flow rate of exhaust gas during the DPF forcible regeneration, and an injector diagnosing device including a heat-generation region determining unit to which detection values of the temperature sensors and a determination value of the SV ratio determining device are input to determine whether or not these values are within a theoretical heat-generation region, and an actual injection amount diagnosing unit that diagnoses a decrease amount in an actual injection amount of an exhaust pipe injector, when the detection values of the temperature sensors and the determination value of the SV ratio determining device are within the theoretical heat-generation region.