high altitude the bellows 78 is expanded to close the outlet port 80 at its lower end as shown in fig4 as a result of reduction in the atmospheric pressure as a result the supply of the bleed air to the second communication passage 114 is ceased so that the intake vacuum transmitted from the intake pipe 10 solely acts in the second diaphragm chamber 104 in consequence the choke lever 102 is moved to the left by the force of the intake vacuum overcoming the force of the spring 108 the leftward movement of the lever 102 causes a counter clockwise rotation of the arm 23a so that the retainer portion 23b comes into contact with the projection 20b to increase the opening degree of the choke valve 14 thus the choke valve 14 can have a larger opening degree at high altitude than at low altitude so that the undesirable enrichment of air fuel mixture at high altitude is avoided advantageously in the second embodiment described hereinbefore the lower end 79 of the bellows 78 is shaped in a conical form so that the opening area of the outlet port 80 is progressively decreased as the bellows expands gradually in.