An air blowing device, comprising: a casing having an outside air inlet (10c, 10d) through which air outside a cabin is introduced as air-conditioning air, and an inside air inlet (10a, 10b) through which air inside the cabin is introduced as the air-conditioning air; an inside/outside air switching door (20, 21) which opens and closes the outside air inlet (10c, 10d) and the inside air inlet (10a, 10b); and a door drive unit which drives the inside/outside air switching door (20, 21), the air blowing device being mounted on a vehicle, and blowing the air-conditioning air, wherein the casing is divided into a front casing member and a rear casing member at a center portion of the casing in the lengthwise direction of the vehicle, a filter insertion hole through which a filter for filtering the air-conditioning air is inserted into the casing is formed through a rear wall of the rear casing member, the outside air inlet (10c, 10d) and the inside air inlet (10a, 10b) of the casing extend in a lengthwise direction of the vehicle, the inside/outside air switching door (20, 21) has a rotating shaft which extends in the lengthwise direction of the vehicle, and is rotatably supported by the casing, the rotating shaft being provided with an open/close plate which opens and closes the outside air inlet (10c, 10d) and the inside air inlet (10a, 10b), the door drive unit is attached to a portion of the rear wall of the rear casing member, the portion being positioned above the filter insertion hole, and the door drive unit is coupled to the rotating shaft of the inside/outside air switching door (20, 21), a first passage and a second passage, through each of which the air-conditioning air flows, are arranged one above the other in a vertical direction in the air blowing device, each of the first and second passages communicating with the outside air inlet (10c, 10d) and the inside air inlet (10a, 10b), and a first fan and a second fan, each of which blows the air-conditioning air, are respectively arranged in the first and second passages. a casing having an outside air inlet (10c, 10d) through which air outside a cabin is introduced as air-conditioning air, and an inside air inlet (10a, 10b) through which air inside the cabin is introduced as the air-conditioning air; an inside/outside air switching door (20, 21) which opens and closes the outside air inlet (10c, 10d) and the inside air inlet (10a, 10b); and a door drive unit which drives the inside/outside air switching door (20, 21), the air blowing device being mounted on a vehicle, and blowing the air-conditioning air, wherein the casing is divided into a front casing member and a rear casing member at a center portion of the casing in the lengthwise direction of the vehicle, a filter insertion hole through which a filter for filtering the air-conditioning air is inserted into the casing is formed through a rear wall of the rear casing member, the outside air inlet (10c, 10d) and the inside air inlet (10a, 10b) of the casing extend in a lengthwise direction of the vehicle, the inside/outside air switching door (20, 21) has a rotating shaft which extends in the lengthwise direction of the vehicle, and is rotatably supported by the casing, the rotating shaft being provided with an open/close plate which opens and closes the outside air inlet (10c, 10d) and the inside air inlet (10a, 10b), the door drive unit is attached to a portion of the rear wall of the rear casing member, the portion being positioned above the filter insertion hole, and the door drive unit is coupled to the rotating shaft of the inside/outside air switching door (20, 21), a first passage and a second passage, through each of which the air-conditioning air flows, are arranged one above the other in a vertical direction in the air blowing device, each of the first and second passages communicating with the outside air inlet (10c, 10d) and the inside air inlet (10a, 10b), and a first fan and a second fan, each of which blows the air-conditioning air, are respectively arranged in the first and second passages.