An air conditioner for use with a single-component refrigerant mainly comprising hydrofluoroolefin having a carbon-carbon double bond or a mixture refrigerant containing this refrigerant as a fluid, the air conditioner comprising: an hermetic compressor, a condenser (33, 36), a pressure-reducing device and an evaporator (33, 36), all connected in series by refrigerant piping to define a refrigerating cycle, the hermetic compressor comprising: a closed container having a suction port and a discharge port both defined therein; a compression mechanism operable to compress a refrigerant inhaled through the suction port; a high-pressure chamber operable to communicate the compression mechanism with the discharge port; a lubricating oil sump disposed at a bottom portion of the high-pressure chamber; a lubricating oil supply means operable to supply lubricating oil from the lubricating oil sump to the compression mechanism; and a heat source (20, 24) operable to heat the lubricating oil, characterized in that the air conditioner further comprises a control device operable to control power supply to the heat source (20, 24) and an inverter circuit operable to drive the hermetic compressor, wherein the hermetic compressor has a variable speed and the control device controls power supply to the heat source (20, 24) based on power consumption or an electric current of the hermetic compressor or an output frequency of the inverter circuit. an hermetic compressor, a condenser (33, 36), a pressure-reducing device and an evaporator (33, 36), all connected in series by refrigerant piping to define a refrigerating cycle, the hermetic compressor comprising: a closed container having a suction port and a discharge port both defined therein; a compression mechanism operable to compress a refrigerant inhaled through the suction port; a high-pressure chamber operable to communicate the compression mechanism with the discharge port; a lubricating oil sump disposed at a bottom portion of the high-pressure chamber; a lubricating oil supply means operable to supply lubricating oil from the lubricating oil sump to the compression mechanism; and a heat source (20, 24) operable to heat the lubricating oil, a closed container having a suction port and a discharge port both defined therein; a compression mechanism operable to compress a refrigerant inhaled through the suction port; a high-pressure chamber operable to communicate the compression mechanism with the discharge port; a lubricating oil sump disposed at a bottom portion of the high-pressure chamber; a lubricating oil supply means operable to supply lubricating oil from the lubricating oil sump to the compression mechanism; and a heat source (20, 24) operable to heat the lubricating oil, characterized in that the air conditioner further comprises a control device operable to control power supply to the heat source (20, 24) and an inverter circuit operable to drive the hermetic compressor, wherein the hermetic compressor has a variable speed and the control device controls power supply to the heat source (20, 24) based on power consumption or an electric current of the hermetic compressor or an output frequency of the inverter circuit.