To provide a controller for a ventilation device which gives superior amenity and has superior energy-saving characteristics.

A controller 10 includes an acquisition part 12, an operation part 13, and a determination part 14. The acquisition part 12 acquires information on an outside air temperature OT, an indoor temperature IT, and a set temperature SP of an air conditioner 2. The operation part 13 calculates an air conditioner power consumption reduction quantity ΔAE and a ventilation device power consumption ΔVE of a ventilation device 3 in outside air cooling or outside air heating. The determination part 14 determines to carry out the outside air cooling or outside air heating when the air conditioner power consumption reduction quantity ΔAE is larger than the ventilation device power consumption ΔVE in the case that the outside air temperature OT, the indoor temperature IT, and set temperature SP meet predetermined conditions.