To inexpensively reduce CO, HC, NOx, CO2, PM in exhaust gas and improve output through easy installation to an existing internal combustion engine by using heat of a humidifier and the internal combustion engine, and to prevent rust and corrosion caused by water droplets by humidifying.

An intake air adjusting device can be easily mounted since it is mounted not inside an intake air introducing pipe of the internal combustion engine but around an intake air taking-in port. In addition, it is not required to modify the intake air introducing pipe etc. of the existing internal combustion engine. The intake air adjusting device appropriately adjusts air amount, temperature and humidity based on the states of the internal combustion engine and exhaust gas, and sends air to the intake air taking-in port. Due to this, CO, HC, NOx, CO2, PM in exhaust gas can be reduced to improve output, and rust and corrosion caused by water droplets by humidifying can be prevented.