PURPOSE: To enable a mixture of fixed air-fuel ratio to be supplied only by adjusting a bypass air amount, by connecting a slow speed fuel system with a bypass passage through a communication path and changing a negative pressure, generated in the bypass passage, acting on the slow speed fuel system.

CONSTITUTION: An air regulating valve 27 is advanced so as to block the outlet end of a bypass passage 23 to an intake path 2 by screwing an adjusting screw thread 28, and if a bypass air flow is decreased to a small amount, a negative pressure in a slow port chamber 22 is generated higher than that in an air chamber 24 formed in the halfway of the bypass passage 23. Consequently, air is allowed to flow into the slow port chamber 22 from the air chamber 24 passing through a communication path 29, and a flow of fuel, sucked to an intake path 2 from a slow port 16, decreases to a less amount. While the air regulating valve 27 is retracted, and if the bypass air flow is increased, the fuel is allowed to flow into the bypass passage 23 from the communication path 29 because a negative pressure is generated in the air chamber 24 higher than that in the slow port chamber 22, here the fuel, added to the fuel sucked from the slow port 16, is supplied to an engine.