In an air flow rate control apparatus having an idle revolution speed control apparatus, in order to prevent the measuring part from freezing by water drops in the idle revolution speed control apparatus, and in order to layout an engine room easily and freely, a perpendicular cavity 9 of cylinder type is formed in a base part 2B of a throttle body 2, in which a bottom part of the cavity 9 is closed by a seal plug 7 and an upper part of the cavity 9 is led to a boa 3 of a cylinder portion 2A, and a bottom par of the perpendicular cavity 9 is arranged lower than a bottom end of aperture part 10 of an air passage and a water pool portion 9B is formed thereunder. Furthermore, a passage section lower part of an air-outlet passage 22a, 22b of a ISC valve 5 is arranged lower than the measuring part 39a, 39b of the bottom end. The water in the measuring part of the ISC valve 5 is pooled in the water pool portion 9B while the engine stops, and the water in the water pool portion is absorbed so as to be drained by a cranking negative pressure through the boa 3 of the throttle body.