To provide an intake air humidifying device using water, which can be installed on an internal combustion engine at low cost to improve output, and reduce exhaust gas, which can be installed on a currently used car at low cost to adapt it to regulations of an automotive exhaust gas regulation NOx and PM law, and in which bad effects of rust and corrosion due to use of water are not given to the internal combustion engine.

This device is installed on an intake air intake port of the internal combustion engine to humidify intake air, improve output of the internal combustion engine, and reduce exhaust gas. In accordance with the state of the internal combustion engine and the state of outside air, humidifying quantity by the humidifying device 27 and humidified air quantity by a humidified air adjusting valve 25 are adjusted to feed the optimum humidified air to the internal combustion engine. Due to simple structure and low cost, it can be easily installed on the internal combustion engine in a currently used car for reducing NOX in exhaust. Since humidified air is sucked by suction force of the internal combustion engine to prevent a high humidity state, bad effects such as rust and corrosion will not be given to the internal combustion engine.