An evaporative emission control system is provided which includes a bypass valve, purge control valve and a vent control valve. To make a leak diagnosis, firstly the vent control valve is closed while the purge control valve is opened, whereby to negatively pressurize the inside of a fuel vapor conduit portion extending from the bypass valve to an intake pipe side. After the pressure in the fuel vapor conduit portion from the bypass valve to the intake pipe is reduced to a predetermined value, the purge control valve is closed. Then, the bypass valve is opened, and after lapse of a predetermined time from the opening of the bypass valve a variation of pressure in a conduit extending from the fuel tank to the purge control valve is asured. Based on the variation of pressure, a leak diagnosis is made. By this, the evaporative emission control system can prevent the fuel vapor in the fuel tank from being drawn into the intake pipe during reduction of pressure in the conduit and thus causing unstable operation of an engine.