A DPF system for performing a DPF forcible regeneration that combusts and removes PMs deposited in a DPF by injecting fuel from an exhaust pipe injector based on an indication-injection amount and oxidizing and combusting the injected fuel with a DOC, the DPF system comprising: temperature sensors (26, 27) that are provided on an entrance side and an exit side of the DOC to detect a DOC entrance temperature and a DOC exit temperature during the DPF forcible regeneration; a SV ratio determining means that determines an exhaust gas SV ratio as ratio of the flow rate of the exhaust gas to a volume of the DOC by measuring flow rate of exhaust gas during the DPF forcible regeneration; and an injector diagnosing means; characterized in that the injector diagnosing means includes: a heat-generation region determining unit to which detection values of the temperature sensors (26, 27) and a determination value of the SV ratio determining means are input, the heat-generation region determining unit being adapted to determine whether or not these values are within an theoretical heat-generation region in which the fuel added from the exhaust pipe injector is completely combusted with the DOC, wherein the theoretical heat-generation region is set based on the exhaust gas SV ratio and the DOC entrance temperature; and an actual injection amount diagnosing unit that is adapted to diagnose a decrease in an actual injection amount of the exhaust pipe injector from the indication-injection amount, when the detection values of the temperature sensors (26, 27) and the determination value of the SV ratio determining means are within the theoretical heat-generation region but not when these values are outside the theoretical heat-generation region. temperature sensors (26, 27) that are provided on an entrance side and an exit side of the DOC to detect a DOC entrance temperature and a DOC exit temperature during the DPF forcible regeneration; a SV ratio determining means that determines an exhaust gas SV ratio as ratio of the flow rate of the exhaust gas to a volume of the DOC by measuring flow rate of exhaust gas during the DPF forcible regeneration; and an injector diagnosing means; characterized in that the injector diagnosing means includes: a heat-generation region determining unit to which detection values of the temperature sensors (26, 27) and a determination value of the SV ratio determining means are input, the heat-generation region determining unit being adapted to determine whether or not these values are within an theoretical heat-generation region in which the fuel added from the exhaust pipe injector is completely combusted with the DOC, wherein the theoretical heat-generation region is set based on the exhaust gas SV ratio and the DOC entrance temperature; and an actual injection amount diagnosing unit that is adapted to diagnose a decrease in an actual injection amount of the exhaust pipe injector from the indication-injection amount, when the detection values of the temperature sensors (26, 27) and the determination value of the SV ratio determining means are within the theoretical heat-generation region but not when these values are outside the theoretical heat-generation region. a heat-generation region determining unit to which detection values of the temperature sensors (26, 27) and a determination value of the SV ratio determining means are input, the heat-generation region determining unit being adapted to determine whether or not these values are within an theoretical heat-generation region in which the fuel added from the exhaust pipe injector is completely combusted with the DOC, wherein the theoretical heat-generation region is set based on the exhaust gas SV ratio and the DOC entrance temperature; and an actual injection amount diagnosing unit that is adapted to diagnose a decrease in an actual injection amount of the exhaust pipe injector from the indication-injection amount, when the detection values of the temperature sensors (26, 27) and the determination value of the SV ratio determining means are within the theoretical heat-generation region but not when these values are outside the theoretical heat-generation region.