An intake air amount controlling apparatus of an engine is provided. The apparatus comprises an intake manifold that is branched from an intake manifold collecting portion of the engine. The intake manifold communicates with an intake port of a cylinder head of a corresponding cylinder. The apparatus further comprises a throttle valve in the intake manifold of the cylinder and a throttle bore provided in the intake manifold. The throttle bore has a curved surface. The throttle valve rotates such that one half of the throttle valve contacts with the curved surface of the throttle bore until the throttle valve reaches a predetermined opening degree. Thus, an air flow via the one half of the throttle valve is prevented from entering the intake port. An air flow via the other half of the throttle valve is allowed to enter the intake port. Areas of cross sections, which are orthogonal to an air flow passing in the intake manifold, of the intake manifold at upstream and downstream of the throttle valve are larger than an area of a cross section, which is orthogonal to an air flow entering the intake port, crossing an open face at which the intake manifold communicates with the intake port.