Fuel supply control system for internal combustion engine

A fuel supply control system is arranged to execute a first diagnosis based on the a fuel injection quantity (Qi) and a fuel pump discharge quantity (Q) after a feedback control of a fuel pressure (p) is started, to execute a second diagnosis when a result of the first diagnosis is NG or when the fuel pump discharge quantity (Q) approximately takes one of a maximum quantity (MAX) or minimum quantity (MIN) of the fuel pump discharge quantity, and to make a NG decision in the second diagnosis when a condition of a magnitude of a difference (Pcal) between a target fuel pressure (Pt) and an actual fuel pressure (P) is greater than a predetermined pressure is maintained for a predetermined time period (T).