An apparatus for detecting a malfunction in an evaporated fuel purge system, includes a diagnosis control valve provided in an air inlet passage connecting an air inlet of a canister to the atmosphere, a pressure sensor provided at an intermediate portion of a vapor passage between a fuel tank and a purge control valve for sensing a pressure in the vapor passage, and a control part for controlling opening and closing operations of both the purge control valve and the diagnosis control valve. In this apparatus, the purge control valve is opened and the diagnosis control valve is closed by the control part under a predetermined operating condition of an engine, the purge control valve being closed after the vapor passage is subjected to a negative pressure in an intake passage of the engine, the negative pressure being maintained during a predetermined time period, the apparatus thus detecting whether or not a malfunction has occurred in the system, in response to a difference between pressures sensed by the pressure sensor respectively at a beginning of the predetermined time period and at an end thereof.