An indoor unit (10) has an indoor side transmission/reception section (15) that is connected to a signal line (L3) and uses a photocoupler and has an indoor control section (12) that is connected between a power source line (L1) and the signal line (L3) and controls, in standby, a relay (MR10) based on outputs from the relay (MR10) in an open state and from the indoor side transmission/reception section (15). An outdoor unit (20) has an outdoor side transmission/reception section (25) that is connected to the signal line (L3) and uses a photocoupler and has a power source circuit (23) to which electric power is supplied from two power source lines (L1, L2). When the indoor control section (12) of the indoor unit (10) starts operation from a standby state at which the power supply to the outdoor unit (20) is stopped, the indoor control section does not bring the relay (MR10) into a closed state when the indoor control section determines, based on a reception output of the indoor side transmission/reception section (15), that a connection wiring connecting the indoor unit (10) and an outdoor unit is faulty.