To provide an intake air heating system for improving efficiency in response to operation of an engine, and to inexpensively provide a device causing no deterioration of engine performance at switching time of an operation state in the device for controlling the temperature of intake air so as to become the normal temperature at high load and accelerating operation time.

This intake air heating system comprises a first opening-closing mechanism capable of adjusting the mixing ratio of heating intake air and outside air on the intake pipe side of the engine, and a second opening-closing mechanism capable of adjusting the mixing ratio of the heating intake air and the outside air on the heating space side by arranging a heating space formed so as to surround an exhaust manifold of the engine. The intake air heating system is constituted so as to prevent a temperature of the heating intake air from the heating space side from rising more than necessary to prevent the deterioration of the engine performance caused by a measuring time delay of a temperature sensor. The intake air heating system can be constituted of an inexpensive material having low heat resistance.