PURPOSE: To atomize fuel well even at the time of the rise in the degree of opening of a throttle valve to improve the distribution of the fuel in a cylinder, by providing a tapered restrictor whose downstream end is shaped as a sharp edge along the total circumference and which is located immediately under a heater for unatomized fuel in a mixture and constricts a mixture passage.

CONSTITUTION: When fuel is supplied, most of it cannot follow the flow of air, so that most of the fuel clings to the top of a throttle valve 1a and the inside of the intake passage of a throttle casing 1 and then flows down as liquid films. When the fuel clinging to the top of the valve 1a goes off the peripheral edge of the valve, the fuel is atomized by the air flow. The liquid film on the inside of the intake passage flows down to the inside of a heating pipe 3a. When a switch is turned on, a PTC ceramic element instantaneously reaches a temperature around 120°C to heat the pipe 3a. When the degree of opening of the throttle valve is heightened, unatomized fuel is mixed with the air to promote the atomization of the fuel. Other fuel not mixed with the air is mixed with it and atomized when the fuel drops down from the downstream end 4a of a restrictor 4. Since the downstream end 4a is located in the center of a chamber 2c, the atomized fuel does not cling again to a wall surface.