In a failure diagnostic system for a fuel vapor processing apparatus, a processing flow path comprising a vapor line, a canister and a purge line is closed with a vapor control valve and a purge control valve. The interior of the processing flow path is reduced in pressure or pressurized by rotating an air pump of a pump module in a forward or reverse direction. An electronic control unit (ECU) closes the vapor control valve and the purge control valve to close the processing flow path after stop of the engine and, when the fuel temperature detected by a temperature sensor is lower than a predetermined value, reduces the internal pressure of the processing flow path through an air pump, then checks air-tightness of the interior of the processing flow path based on behavior of a vapor pressure detected by a pressure sensor. The ECU closes the processing flow path and, when the fuel temperature detected is not lower than the predetermined value, pressurizes the interior of the processing flow path through the air pump, then checks air-tightness of the processing flow path based on behavior of a vapor pressure subsequently detected.