Heat source unit for an air conditioner comprising a refrigerant circuit, the heat source unit comprising: an external housing accommodating: a compressor to be connected to the refrigerant circuit; a heat source heat exchanger to be connected to the refrigerant circuit and configured to exchange heat between a refrigerant circulating in the refrigerant circuit and a heat source; an electric box having a top and side walls (32 to 34), the electric box accommodating electrical components configured to control the air conditioner, a cooling heat exchanger being connected to a bypass line branched from a liquid refrigerant line and a gas suction line, wherein the bypass line has a valve, preferably a solenoid valve, upstream of the cooling heat exchanger; and a controller configured to control the valve in an OFF-mode in which the valve is closed and an ON-mode in which the valve is opened, characterized in that the electric box has an air passage comprising an air inlet and an air outlet, an air flow being induced through the air passage from the air inlet to the air outlet for cooling at least some of the electrical components, and the cooling heat exchanger is accommodated in the external housing to be connected to the refrigerant circuit, wherein the cooling heat exchanger is arranged so as to be flown through by the air flow and to exchange heat between the refrigerant and the air flow. an external housing accommodating: a compressor to be connected to the refrigerant circuit; a heat source heat exchanger to be connected to the refrigerant circuit and configured to exchange heat between a refrigerant circulating in the refrigerant circuit and a heat source; an electric box having a top and side walls (32 to 34), the electric box accommodating electrical components configured to control the air conditioner, a cooling heat exchanger being connected to a bypass line branched from a liquid refrigerant line and a gas suction line, wherein the bypass line has a valve, preferably a solenoid valve, upstream of the cooling heat exchanger; and a controller configured to control the valve in an OFF-mode in which the valve is closed and an ON-mode in which the valve is opened, a compressor to be connected to the refrigerant circuit; a heat source heat exchanger to be connected to the refrigerant circuit and configured to exchange heat between a refrigerant circulating in the refrigerant circuit and a heat source; an electric box having a top and side walls (32 to 34), the electric box accommodating electrical components configured to control the air conditioner, a cooling heat exchanger being connected to a bypass line branched from a liquid refrigerant line and a gas suction line, wherein the bypass line has a valve, preferably a solenoid valve, upstream of the cooling heat exchanger; and a controller configured to control the valve in an OFF-mode in which the valve is closed and an ON-mode in which the valve is opened, characterized in that the electric box has an air passage comprising an air inlet and an air outlet, an air flow being induced through the air passage from the air inlet to the air outlet for cooling at least some of the electrical components, and the cooling heat exchanger is accommodated in the external housing to be connected to the refrigerant circuit, wherein the cooling heat exchanger is arranged so as to be flown through by the air flow and to exchange heat between the refrigerant and the air flow.