PURPOSE: To realize a fuel additive supply device for supplying a fuel additive to the fuel system of an internal combustion engine for an automobile in a simple constitution and at a low cost without electric control.

CONSTITUTION: An additive tank 1 for storing an additive is provided in the vicinity of a supply port of a fuel passage 3 to a fuel tank, the additive tank 1 and the fuel passage 3 are connected to each other by a communicating path 2, and an additive measuring chamber 14 is disposed in the midway of the communicating path 2. A non-return valve 17 is provided on the upstream side of the additive measuring chamber 14, and a pressure valve 16 is provided on the downstream side thereof. A piston 13 is disposed in the additive measuring chamber 14, and the piston 13 is connected to a fuel lid 4 by a link 43. The piston 13 is pressed by opening the fuel lid 4 to pressurize the interior of the additive measuring chamber 14 and discharge the additive. The piston 13 is pulled by closing the fuel lid 4 to reduce the pressure in the additive measuring chamber 14 and to draw the additive from the additive tank 1 into the measuring chamber 14.