PURPOSE: To control an air-fuel mixture over the whole output scope by providing a group of second throttles arranged symmetrically to a vertical axis for an axis of a group of first throttles on the downstream side of the group of the first throttles in a guide passage.

CONSTITUTION: Auxiliary gas is supplied to a passage 27 through an inlet 5 and a throttle valve 6, flows around a group of throttle walls 38, and is sent to an intake passage 23 via a group of first throttles 8 so that effective and uniform air-fuel mixture is obtained from an idling speed to a partial load scope. When the auxiliary gas exceeds an average partial load and reaches a full load condition, however, the auxiliary gas is sent into a passage of a second device 4 through an inlet 25, flows around a wall of throttle walls 39, overrides a group of related inclined faces 10, passes a group of second throttles 9, and flows into the intake passage 23. Since an axis 34 of the group of throttles 8 forms a right angle for an axis 35 of the group of throttles 9 and a total of angles of opening holes 24, 28 of the groups of throttles 8, 9, are about 360°, complete atomization of condensed air is done at whole inner periphery of the passage 23.