To diagnose linearity of an output characteristic of a rail pressure sensor by a simple method, and to minimize limitation in working conditions.

A maximum pressure obtained by a rail pressure sensor is regarded as a maximum pressure in opening a pressure limiting valve 10 while fully opening a metering valve 6, then a pressure obtained by the rail pressure sensor 11 is regarded as a high flow rate-side secondary pressure in a state that the pressure limiting valve 10 is opened, and a high-pressure pump 7 is in a full force-feeding state, then a valve opening of the metering valve 6 is reduced, so that a state in which a passing flow rate of the pressure limiting valve 10 is made to be smaller than a passing flow rate in the full force-feeding state of the high-pressure pump 7 and a valve opening state of the pressure limiting valve 10 can be kept is achieved, and a pressure obtained by the rail pressure sensor 11 at that time is regarded as a low flow rate-side secondary pressure. The failure of the rail pressure sensor 11 is determined when any of the maximum pressure, the high flow rate-side secondary pressure and the low flow rate-side secondary pressure is out of a predetermined acceptable measurement range.