A fuel supply system for an internal combustion engine has a fuel pump capable of pumping fuel from a fuel tank to the internal combustion engine, a motor driving the fuel pump, a pressure sensor configured to detect pressure of the fuel pumped from the fuel pump, and a control unit configured to output control signals to the motor. The control unit is configured to perform feedback control of duty ratio of the control signals for the motor, and to control the duty lower limit guard value in order to limit an amount of decreases of the duty lower limit guard value in the step-like manner when a duty lower limit guard value decreases.