PURPOSE: To provide enough engine performance by branching the downstream side of a bypass air introduction passage into two branch passages through a switching valve, effecting switching control according to the operation state of an engine, and forming the one for improvement of startability at a very low temperature period and the other for stabilization of idle rotation.

CONSTITUTION: When an engine is started at a very low temperature period, a control part 20 disconnects energization to a solenoid 40. Thus, a valve body 34 of a switching valve 18 opens a first branch passage 14 obliquely connected to an intake passage 10 through the force of a spring 36 and switches a second branch passage 16 to a closing position. This switching causes introduction of bypass air through the first branch passage 14 to the second intake passage 10 of an intake manifold 6. Thus, starting is improved. During idle rotation, the control device 20 switches the switching valve 18 to the side, where the second branch passage 16 connected to the intake passage 10 in a right angled direction is opened, based on signals from sensors 44-50, such as a water temperature sensor 46. In this way, bypass air is introduced through the second branch passage 16 to the intake passage 10 to stabilize idle rotation.