PROBLEM TO BE SOLVED: To provide a device for reducing aldehyde in a passenger compartment, capable of continuously reducing the concentration of aldehyde in air in the passenger compartment to a desired concentration or lower without the need to replace a filter or the like.SOLUTION: An aldehyde reduction device 1 comprises: a gas sensor 5 detecting an aldehyde concentration Qr in air in a passenger compartment 4 of a vehicle 2 a humidification unit 7 humidifying the passenger compartment 4 an air-conditioning unit 17 cooling the air in the passenger compartment 4 and a control unit 31. If the aldehyde concentration Qr of the passenger compartment 4 detected by the gas sensor 5 exceeds a first threshold Qh1, the control unit 31 actuates the humidification unit 7 to humidify the air in the passenger compartment 4 and to increase a quantity of water vapor, and increases a quantity of aldehyde captured by the water vapor. Furthermore, the control unit 31 actuates the air-conditioning unit 17 to cool the humidified air in the passenger compartment 4 and to condense the water vapor, and discharges condensate water containing aldehyde to outside of the passenger compartment 4.
