In an air intake system, a rear tower having an interior space is provided on the top surface of a body frame. A rear pillar having an interior passageway is mounted on the rear tower. An air intake port communicating with the passageway is provided in the rear pillar, and the passageway and the space within the rear tower are connected together. An expansion silencer is connected to the space of the rear tower and an air cleaner is connected to the expansion silencer. The top surface of a working oil tank is lower than the upper end of a side plate to form a space above the top surface, where the expansion silencer is provided. A base bracket is mounted to the air cleaner, a support bracket is mounted on the side plate, and the base bracket is mounted on the support bracket to support the air cleaner above the expansion silencer.