A shut-off valve fault diagnosis device comprising: a shut-off valve including a first valve body that is opened in advance during power supply and a second valve body that is opened due to a decrease in a differential pressure between upstream and downstream after the first valve body is opened; and a diagnosis processing unit that estimates an open and shut state of each of the first and second valve bodies (103, 104) from relationship between a time-variable characteristic in an actual measurement value of a downstream pressure of the shut-off valve and a predetermined threshold value, and performs the fault diagnosis of the shut-off valve based on the estimation result, wherein in a case where the actual measurement value of the downstream pressure before the power supply to the shut-off valve is less than or equal to the threshold value, the diagnosis processing unit determines whether or not the actual measurement value of the downstream pressure exceeds the threshold value after a predetermined time has passed from initiation of the power supply to the shut-off valve, and in a case where it is determined that the actual measurement value does not exceed the threshold value, the diagnosis processing unit estimates that at least the first valve body is in a shut valve state and determines that the shut-off valve is in a fault state, and wherein in a case where it is determined that the actual measurement value of the downstream pressure exceeds the threshold value after a predetermined time has passed from the initiation of the power supply to the shut-off valve, the diagnosis processing unit activates a fuel injection valve provided downstream of the shut-off valve, and in a case where the actual measurement value of the downstream pressure becomes less than or equal to the threshold value after the activation of the fuel injection valve, the diagnosis processing unit estimates that the first valve body is in an open valve state and the second valve body is in a shut valve state and determines that the shut-off valve is in a fault state, and/or wherein in a case where the actual measurement value of the downstream pressure exceeds the threshold value before the power supply to the shut-off valve,, the diagnosis processing unit activates a fuel injection valve provided downstream of the shut-off valve after the initiation of the power supply to the shut-off valve, and in a case where the actual measurement value of the downstream pressure is less than or equal to the threshold value after the activation of the fuel injection valve, the diagnosis processing unit estimates that at least the second valve body is in a shut valve state and determines that the shut-off valve is in a fault state. a shut-off valve including a first valve body that is opened in advance during power supply and a second valve body that is opened due to a decrease in a differential pressure between upstream and downstream after the first valve body is opened; and a diagnosis processing unit that estimates an open and shut state of each of the first and second valve bodies (103, 104) from relationship between a time-variable characteristic in an actual measurement value of a downstream pressure of the shut-off valve and a predetermined threshold value, and performs the fault diagnosis of the shut-off valve based on the estimation result, wherein in a case where the actual measurement value of the downstream pressure before the power supply to the shut-off valve is less than or equal to the threshold value, the diagnosis processing unit determines whether or not the actual measurement value of the downstream pressure exceeds the threshold value after a predetermined time has passed from initiation of the power supply to the shut-off valve, and in a case where it is determined that the actual measurement value does not exceed the threshold value, the diagnosis processing unit estimates that at least the first valve body is in a shut valve state and determines that the shut-off valve is in a fault state, and wherein in a case where it is determined that the actual measurement value of the downstream pressure exceeds the threshold value after a predetermined time has passed from the initiation of the power supply to the shut-off valve, the diagnosis processing unit activates a fuel injection valve provided downstream of the shut-off valve, and in a case where the actual measurement value of the downstream pressure becomes less than or equal to the threshold value after the activation of the fuel injection valve, the diagnosis processing unit estimates that the first valve body is in an open valve state and the second valve body is in a shut valve state and determines that the shut-off valve is in a fault state, and/or wherein in a case where the actual measurement value of the downstream pressure exceeds the threshold value before the power supply to the shut-off valve,, the diagnosis processing unit activates a fuel injection valve provided downstream of the shut-off valve after the initiation of the power supply to the shut-off valve, and in a case where the actual measurement value of the downstream pressure is less than or equal to the threshold value after the activation of the fuel injection valve, the diagnosis processing unit estimates that at least the second valve body is in a shut valve state and determines that the shut-off valve is in a fault state.