The error diagnosis device has: an input unit that receives detected values for pressure inside the common rail; a calculation unit that, if the detected values are less than a target common rail pressure, calculates the pressure differences and the fuel pump discharge amounts; and a determination unit that determines whether or not a set time has lapsed in a state in which the pressure differences are at least a first threshold value and less than a second threshold value and the discharge amounts are at least a third threshold value and less than a fourth threshold value. The determination unit determines that an error has occurred in the high-pressure pump if the set time has lapsed and determines that an error has occurred in the flowrate adjustment valve if the set time has not lapsed.