To provide a fuel supply device for an internal combustion engine capable of suppressing adverse effects by alcohol fuel of high concentration on component members supplying fuel to a fuel injection valve.

Fuel including an alcohol component is stored in a fuel tank 9. A pressure sensor 35 for detecting fuel pressure is provided in a delivery pipe 14 to which fuel inside the fuel tank 9 is supplied. A diluent tank 50 storing diluent of alcohol fuel is connected to the fuel tank 9. The supply amount of the diluent to the fuel tank 9 is adjusted by a water injection valve 52. A control device 40 estimates alcohol concentration of the fuel inside the fuel tank 9 based on the speed of fuel pressure increase at high-temperature start of the engine, and calculates addition amount of the diluent based on the estimated alcohol concentration. Then, the control device 40 sets opening time of the water injection valve 52 based on the calculated addition amount, and performs opening control of the water injection valve 52.