To provide an idling air amount adjusting device capable of giving a fast idling air amount increased by a certain amount from an adjusted normal idling air amount.

An idling air amount adjusting device 40 includes a plunger 42 capable of push-pull operation and rotation operation, a valve member 64 which can change its position by the push-pull operation of the plunger 42 and can adjust its position by the rotation operation of the plunger 42. The valve member 64 opens a bypass passage 34 and can adjust an open area of the bypass passage 34 so that the idling air amount agrees with a normal idling air amount at a push position of an operation member, and opens the bypass passage 34 so that the air amount increased by a certain amount from the adjusted normal idling air amount agrees with a fast idling air amount at a pull position of the operation member.