To provide a technology capable of reducing energy costs while keeping work environment in a ventilation-needed space.

A ventilating section 48 exhausts the indoor air. An air supply section 50 supplies outside air indoors. An estimating section 56 estimates change of a temperature of the indoor air when an amount of the outside air supplied by the air supply section is changed with several patterns. A control section 66 controls the amount of outside air supplied by the air supply section on the basis of one of several patterns selected by an user on the basis of the estimation.