A vehicle air conditioner comprising: a heat pump device including a compressor which compresses a refrigerant, an interior heat exchanger (31, 32) provided inside a vehicle cabin, an exterior heat exchanger provided outside the vehicle cabin, an expansion valve (52, 53), and an exterior blower (37a, 38a) which blows air toward the exterior heat exchanger, the heat pump device being formed by connecting together the compressor, the interior heat exchanger (31, 32), the expansion valve (52, 53), and the exterior heat exchanger via refrigerant piping; and an air-conditioning controller configured to control the heat pump device, the heat pump device being configured to be switched by the air-conditioning controller between multiple operation modes including a heating operation mode and a defrosting operation mode, wherein the vehicle air conditioner further comprises a frosting state detecting means for detecting a frosting state of the exterior heat exchanger, the exterior heat exchanger includes a plurality of heat-transfer pipes which are arranged in line in a predetermined direction and a header tank (33c, 33d) which is connected to an end of the heat-transfer pipes and which extends in the direction in which the heat-transfer pipes are arranged in line, the header tank (33c, 33d) has a refrigerant inlet portion and a refrigerant outlet portion which are provided along the length of the header tank (33c, 33d) so as to be spaced apart from each other, and a partition (33e, 33f, 33g) which partitions the heat-transfer pipes into a plurality of paths such that a refrigerant flow that goes from the refrigerant inlet portion toward the refrigerant outlet portion is formed, the exterior blower (37a, 38a) includes a first fan and a second fan which are arranged horizontally along an air passage plane of the exterior heat exchanger so as to be located closer to the refrigerant inlet portion and the refrigerant outlet portion, respectively, and the air-conditioning controller is configured to switch, if the frosting state detecting means senses, during the heating mode of operation, that the exterior heat exchanger is frosted, the operation modes of the heat pump device into a defrosting operation mode and to stop the first and second fans (37a, 38a), and to activate the first and second fans (37a, 38a) independently of each other according to the degree of defrosting of the exterior heat exchanger, and set the airflow rate of the second fan to be higher than the airflow rate of the first fan, during the defrosting mode of operation. a heat pump device including a compressor which compresses a refrigerant, an interior heat exchanger (31, 32) provided inside a vehicle cabin, an exterior heat exchanger provided outside the vehicle cabin, an expansion valve (52, 53), and an exterior blower (37a, 38a) which blows air toward the exterior heat exchanger, the heat pump device being formed by connecting together the compressor, the interior heat exchanger (31, 32), the expansion valve (52, 53), and the exterior heat exchanger via refrigerant piping; and an air-conditioning controller configured to control the heat pump device, the heat pump device being configured to be switched by the air-conditioning controller between multiple operation modes including a heating operation mode and a defrosting operation mode, wherein the vehicle air conditioner further comprises a frosting state detecting means for detecting a frosting state of the exterior heat exchanger, the exterior heat exchanger includes a plurality of heat-transfer pipes which are arranged in line in a predetermined direction and a header tank (33c, 33d) which is connected to an end of the heat-transfer pipes and which extends in the direction in which the heat-transfer pipes are arranged in line, the header tank (33c, 33d) has a refrigerant inlet portion and a refrigerant outlet portion which are provided along the length of the header tank (33c, 33d) so as to be spaced apart from each other, and a partition (33e, 33f, 33g) which partitions the heat-transfer pipes into a plurality of paths such that a refrigerant flow that goes from the refrigerant inlet portion toward the refrigerant outlet portion is formed, the exterior blower (37a, 38a) includes a first fan and a second fan which are arranged horizontally along an air passage plane of the exterior heat exchanger so as to be located closer to the refrigerant inlet portion and the refrigerant outlet portion, respectively, and the air-conditioning controller is configured to switch, if the frosting state detecting means senses, during the heating mode of operation, that the exterior heat exchanger is frosted, the operation modes of the heat pump device into a defrosting operation mode and to stop the first and second fans (37a, 38a), and to activate the first and second fans (37a, 38a) independently of each other according to the degree of defrosting of the exterior heat exchanger, and set the airflow rate of the second fan to be higher than the airflow rate of the first fan, during the defrosting mode of operation.