An electronic control unit (ECU) obtains outside air humidity information from a sensor disposed outside a vehicle compartment and inside air humidity information from a sensor inside the vehicle compartment. The ECU includes a humidity information obtainer obtaining the inside air humidity information, a humidity change calculator calculating the inside air humidity information as a difference between (i) a physical quantity correlated with humidity before a start of a dehumidification or a humidification and (ii) a physical quantity correlated with humidity after a switch-off of the dehumidification or the humidification, a soak time calculator calculating, based on the difference of the physical quantities, a soak time from the switch-off of the dehumidification or the humidification to a sensor-diag startable time at which a diagnosis of the outside air humidity sensor is startable, and a comparator comparing the outside air humidity information and the inside air humidity information after a lapse of the soak time from the switch-off of the dehumidification or the humidification.