To provide a throttle chamber to suppress the increase rate of an intake air amount based on the opening of an accelerator pedal, relax rapid torque during acceleration, and improve an acceleration feeling.

An inside rotary shaft 3 having a small inner valve 4 is directly coupled to a throttle drum 2 rotated through a wire 1 by operation of an accelerator pedal, a cylindrical outer rotary shaft 6 internally of which an inner rotary shaft 3 is inserted is provided with an outer valve 5. When the throttle drum 2 is rotated, the inner rotary shaft 3 is rotated until a claw 3a arranged at the shaft end of the inner rotary shaft 3 makes contact with a claw 6a or 6b arranged at the shaft end of the outer rotary shaft 6. Only the inner valve 4 is opened and after the claws make contact with each other, the outer rotary shaft 6 is rotated togetherwith the inner rotary shaft 3, and the outer valve 5 is opened togetherwith the inner valve 4.