An air conditioner comprising: an indoor unit having an indoor heat exchanger to evaporate or condense a refrigerant by heat exchange with indoor air; an outdoor unit having a variable capacity compression part for sucking and compressing a refrigerant of a refrigerant intake passage and discharging the refrigerant to a refrigerant discharge passage, a water-refrigerant heat exchanger for condensing or evaporating a refrigerant by heat exchange with heat source water, and an outdoor expansion device (40, 42) installed between the indoor heat exchanger and the water-refrigerant heat exchanger; a heat source water flow path connected to the water-refrigerant heat exchanger; a pump installed on the heat source water flow path; and a variable flow valve installed on the heat source water flow path and capable of regulating the opening degree, characterized by a variable flow valve controller which is configured to control the opening degree of the variable flow valve depending on the pressure of the refrigerant discharge passage during a cooling operation, and to control the opening degree of the variable flow valve depending on the pressure of the refrigerant intake passage during a heating operation, wherein, during the heating operation, if the pressure of the refrigerant intake passage is higher than a target evaporation pressure, the opening degree of the variable flow valve is decreased, and if the pressure of the refrigerant intake passage is lower than the target evaporation pressure, the opening degree of the variable flow valve is increased, wherein the variable flow valve is fully opened upon startup of the cooling operation or the heating operation, and the opening degree is varied upon completion of the startup of the cooling operation or the heating operation. an indoor unit having an indoor heat exchanger to evaporate or condense a refrigerant by heat exchange with indoor air; an outdoor unit having a variable capacity compression part for sucking and compressing a refrigerant of a refrigerant intake passage and discharging the refrigerant to a refrigerant discharge passage, a water-refrigerant heat exchanger for condensing or evaporating a refrigerant by heat exchange with heat source water, and an outdoor expansion device (40, 42) installed between the indoor heat exchanger and the water-refrigerant heat exchanger; a heat source water flow path connected to the water-refrigerant heat exchanger; a pump installed on the heat source water flow path; and a variable flow valve installed on the heat source water flow path and capable of regulating the opening degree, characterized by a variable flow valve controller which is configured to control the opening degree of the variable flow valve depending on the pressure of the refrigerant discharge passage during a cooling operation, and to control the opening degree of the variable flow valve depending on the pressure of the refrigerant intake passage during a heating operation, wherein, during the heating operation, if the pressure of the refrigerant intake passage is higher than a target evaporation pressure, the opening degree of the variable flow valve is decreased, and if the pressure of the refrigerant intake passage is lower than the target evaporation pressure, the opening degree of the variable flow valve is increased, wherein the variable flow valve is fully opened upon startup of the cooling operation or the heating operation, and the opening degree is varied upon completion of the startup of the cooling operation or the heating operation.