PURPOSE: To automatically diagnose the fault of purge operation by providing pressure and fuel sensors in the purge passage system of a canister for evaporated fuel discharge suppressing device and a pressure sensor in the diaphragm chamber pressure system of a purge valve.

CONSTITUTION: The gasoline evaporated portion of a float chamber 12 and a fuel tank 14 is adsorbed in a canister case 10 through a cab pipe 13 and a tank pipe 15 to make the canister case 1 overflow. At the time of engine start, the gasoline component stayed in the canister case 1 is sucked into an engine from a purge passage 6 through an evaporator 11. During this purge operation, the output according to the pressure in a diaphragm chamber pressure passage 4 is detected by a pressure sensor 5, and the output according to the pressure in the purge passage 6 by a pressure sensor 8. Further, the output induced by the concentration in the purge passage 6 is detected by a fuel sensor 8. These output values are processed by a control unit 9, and when a fault is judged, it is noticed by an alarm indicator 10.