To materialize miniaturization in an intake air quantity control device for an engine consisting of a throttle body which includes a throttle bore communicating to a suction port and a throttle valve for adjusting an opening of the throttle bore therein, and a throttle driving means equipped with an electric motor exerting power for drivingly opening and closing the throttle valve and a transmission mechanism for decelerating the driving force of the electric motor to transmit the driving force to the throttle valve.

A driven wheel 48R constituting a part of a timing transmission mechanism 47R for transmitting power from a crankshaft is fixed to one end of a cam shaft 40R included in a valve gear 38R for drivingly opening and closing a suction valve and exhaust valve arranged in a cylinder head 25R so as to allow opening/closing operation. The throttle driving means 70 is arranged on the opposite side to the timing transmission mechanism 47R in a direction along the axial line of the crankshaft. The electric motor 71 is arranged between a throttle body 54B and cylinder head 25R in a plan view.