An indoor unit of an air conditioner comprising an arithmetic unit configured to execute air conditioning control, wherein the arithmetic unit includes: an actuator control unit configured to control an operation of an actuator on the basis of a content of an operation signal transmitted from an operating device; a communication unit configured to communicate with an optional device; an optional device connection determination unit configured to determine whether or not the optional device is connected to the indoor unit of an air conditioner; and a standby power control unit configured to switch processing for reduction of standby power of the arithmetic unit in standby during which the operation of the actuator is stopped, on the basis of a result of the determination on whether or not the optional device is connected, and execute the processing, and the standby power control unit is further configured to perform: control to stop the operation of the arithmetic unit after stopping the operation of the actuator under the control of the actuator control unit, when the optional device connection determination unit determines that the optional device is not connected; characterized in that the standby power control unit is further configured to perform control to set a processing speed of the arithmetic unit during the standby to be lower than a processing speed during operation after stopping the operation of the actuator under the control of the actuator control unit, when the optional device connection determination unit determines that the optional device is connected. an actuator control unit configured to control an operation of an actuator on the basis of a content of an operation signal transmitted from an operating device; a communication unit configured to communicate with an optional device; an optional device connection determination unit configured to determine whether or not the optional device is connected to the indoor unit of an air conditioner; and a standby power control unit configured to switch processing for reduction of standby power of the arithmetic unit in standby during which the operation of the actuator is stopped, on the basis of a result of the determination on whether or not the optional device is connected, and execute the processing, and the standby power control unit is further configured to perform: control to stop the operation of the arithmetic unit after stopping the operation of the actuator under the control of the actuator control unit, when the optional device connection determination unit determines that the optional device is not connected; characterized in that the standby power control unit is further configured to perform control to set a processing speed of the arithmetic unit during the standby to be lower than a processing speed during operation after stopping the operation of the actuator under the control of the actuator control unit, when the optional device connection determination unit determines that the optional device is connected. control to stop the operation of the arithmetic unit after stopping the operation of the actuator under the control of the actuator control unit, when the optional device connection determination unit determines that the optional device is not connected; characterized in that the standby power control unit is further configured to perform control to set a processing speed of the arithmetic unit during the standby to be lower than a processing speed during operation after stopping the operation of the actuator under the control of the actuator control unit, when the optional device connection determination unit determines that the optional device is connected.