PURPOSE: To well atomize fuel and improve combustible performance by forming a radial diffusion portion ranging downward from a nozzle with a nozzle section curved outward and a wall facing a throttle, the wall being provided with a curved portion projecting toward a throttle.

CONSTITUTION: A nozzle 2 has a nozzle inner room limited with its inner wall 3 and tapered downward from its upper section 4 and ranging to the minimum cross section 5. A radial diffusion portion 6 is continued downward from the minimum cross section 5 and the downflow cross section of the diffusion portion 6 is gradually tapered toward the radial exit 19 of the diffusion portion 6 to be moved to a suction pipe 7. In this case, the diffusion portion 6 is formed with a nozzle section curved outward in the flow direction of a mixture, a structure 17 forming a structural unit 18 with the suction pipe 7 and a wall 15 facing a throttle 8, the wall 15 being provided with a curved portion 16 projecting toward the throttle 8.