To provide a fuel supply device for an internal combustion engine having improved combustion stability and exhaust properties by detecting the malfunction of a selector valve, the internal combustion engine having a fuel pressure change-over means for changing over fuel injection pressure with the opening/closing operation of the selector valve.

The internal combustion engine 10 sets the selector valve 34 into an opened condition to make set fuel pressure in a delivery pipe low, and sets it into a closed condition to make the set fuel pressure high. When outputting a command signal to the selector valve 34 to make the set fuel pressure high, an electronic control device 50 diagnoses the malfunction of the selector valve 34 in accordance with the ratio of an actual air-fuel ratio to a target air-fuel ratio at this time. On determining the malfunction of the selector valve 34, it controls the drive of a throttle valve 15 to restrict an intake air amount and suppress the combustion of mixture in a lean condition when a target fuel injection amount each time is greater than the maximum fuel injection amount at each engine speed in the low fuel pressure state.