PURPOSE: To perform appropriate air-fuel ratio compensation by providing bypass passage to flow compensating air by connecting the venturi part upstream and the throttle valve downstream.

CONSTITUTION: The first and the second throttle rings 5, 6 which form the pressure chamber 7 are formed in the bypass passage 4. The electric signals from the venturi negative pressure sensor 12, the atmospheric pressure sensor 13, and the suction pressure sensor 14 are sent to the operation circuit to calculate the pressure in the pressure chamber 7 to obtain the appropriate amount of compensating air in the atmospheric pressure at the moment regardless of the condition of the operation of the internal combustion engine and send the voltage in proportion to the pressure as the electric signal to the comparing circuit. The valve body 8 is shifted on the basis of the electric signal from the comparing circuit 17 to control the opening area of the bypass passage 4.