Heat source unit for an air conditioner comprising: a compressor unit having a compressor disposed in a first casing, first and second heat source heat exchanger unit ports (42, 43) configured to connect the compressor unit to a heat source heat exchanger of at least one heat source heat exchanger unit of the air conditioner, first and second indoor unit ports (46, 47) configured to connect the compressor unit to an indoor heat exchanger of at least one indoor unit of the air conditioner, a first refrigerant piping fluidly connecting the first heat source heat exchanger unit port and the first indoor unit port, a sub-cooling heat exchanger disposed inside the first casing and fluidly connected to the first refrigerant piping for heat transfer with the refrigerant to be flown through the first refrigerant piping, a second refrigerant piping fluidly connecting the second heat source heat exchanger unit port and the second indoor unit port, the compressor and a 4-way valve being interposed between the second heat source heat exchanger unit port and the second indoor unit port in the second refrigerant piping, and a bypass passage connected to the second refrigerant piping at a suction side of the compressor between the compressor and the 4-way valve, the sub-cooling heat exchanger being fluidly connected to the bypass passage for heat transfer between refrigerant flowing in the bypass passage and refrigerant flowing in the first refrigerant piping, and characterized in that further comprising a heat source heat exchanger unit having a heat source heat exchanger, the heat source heat exchanger being disposed in a second casing separate from the first casing and being configured to exchange heat with a heat source, wherein the heat source heat exchanger unit is fluidly connected to the compressor unit via the first and second heat source heat exchanger unit ports (42, 43), wherein a main expansion valve of the air conditioner is disposed in the second casing. a compressor unit having a compressor disposed in a first casing, first and second heat source heat exchanger unit ports (42, 43) configured to connect the compressor unit to a heat source heat exchanger of at least one heat source heat exchanger unit of the air conditioner, first and second indoor unit ports (46, 47) configured to connect the compressor unit to an indoor heat exchanger of at least one indoor unit of the air conditioner, a first refrigerant piping fluidly connecting the first heat source heat exchanger unit port and the first indoor unit port, a sub-cooling heat exchanger disposed inside the first casing and fluidly connected to the first refrigerant piping for heat transfer with the refrigerant to be flown through the first refrigerant piping, a second refrigerant piping fluidly connecting the second heat source heat exchanger unit port and the second indoor unit port, the compressor and a 4-way valve being interposed between the second heat source heat exchanger unit port and the second indoor unit port in the second refrigerant piping, and a bypass passage connected to the second refrigerant piping at a suction side of the compressor between the compressor and the 4-way valve, the sub-cooling heat exchanger being fluidly connected to the bypass passage for heat transfer between refrigerant flowing in the bypass passage and refrigerant flowing in the first refrigerant piping, and characterized in that further comprising a compressor disposed in a first casing, first and second heat source heat exchanger unit ports (42, 43) configured to connect the compressor unit to a heat source heat exchanger of at least one heat source heat exchanger unit of the air conditioner, first and second indoor unit ports (46, 47) configured to connect the compressor unit to an indoor heat exchanger of at least one indoor unit of the air conditioner, a first refrigerant piping fluidly connecting the first heat source heat exchanger unit port and the first indoor unit port, a sub-cooling heat exchanger disposed inside the first casing and fluidly connected to the first refrigerant piping for heat transfer with the refrigerant to be flown through the first refrigerant piping, a second refrigerant piping fluidly connecting the second heat source heat exchanger unit port and the second indoor unit port, the compressor and a 4-way valve being interposed between the second heat source heat exchanger unit port and the second indoor unit port in the second refrigerant piping, and a bypass passage connected to the second refrigerant piping at a suction side of the compressor between the compressor and the 4-way valve, the sub-cooling heat exchanger being fluidly connected to the bypass passage for heat transfer between refrigerant flowing in the bypass passage and refrigerant flowing in the first refrigerant piping, and characterized in that further comprising a heat source heat exchanger unit having a heat source heat exchanger, the heat source heat exchanger being disposed in a second casing separate from the first casing and being configured to exchange heat with a heat source, wherein the heat source heat exchanger unit is fluidly connected to the compressor unit via the first and second heat source heat exchanger unit ports (42, 43), wherein a main expansion valve of the air conditioner is disposed in the second casing.