To provide a ventilation system utilizing nature energy, controlling the temperature of a building by utilizing nature force having combination of both characteristics of solar light and ground heat.

Solar wall side supplied air 13 from a solar wall 3 is detected by a first temperature sensor 28, and ground heat side supplied air 27 from a ground heat tube 16 is detected by a second temperature sensor 29. On the basis of the temperatures detected by the respective temperature sensors 28, 29, the operation of a switching valve and a blower fan 25 is controlled so that the temperature in the building 1 reaches a target temperature, and the solar wall side supplied air 13 or the ground heat side supplied air 27 or mixed air of these is supplied to the interior of the building 1.