A diagnostic system for a fuel system of an internal combustion engine such as direct-injection gasoline engine monitors a pressure deviation of a actual fuel pressure sensed by a fuel pressure sensor from a desired fuel pressure in a feedback fuel pressure control, and thereby detects abnormality in the fuel system. When the pressure deviation continues to be outside a normal range, a diagnostic controller commands engine operation in a homogeneous stoichiometric combustion mode, and monitors a feedback correction quantity in a feedback stoichiometric air fuel control during the engine operation in the homogeneous stoichiometric combustion mode. The controller attributes the abnormality to the fuel pressure sensor if the feedback correction quantity of the air fuel ratio is fixed to an upper limit or a lower limit.