Disclosed is a device for detecting any fixed closure of a fuel shut-off valve 7 which opens/closes a feed line 5 for feed of fuel 3 to a fuel addition valve 6. The device is provided with a pressure sensor 8 which detects a pressure of the fuel 3 between the fuel addition and shut-off valves 6 and 7 and a controller 9 which determines the fuel shut-off valve 7 to be fixed closed when a detection value of the pressure sensor 8 falls below a threshold set depending on a rotation frequency of an engine.