A controller for controlling operation of a heat pump system, the heat pump system comprising: a compressor; a heatsource-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a utilization-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a high-pressure refrigerant pipe connected to each of a discharge port of the compressor and the heatsource-side heat exchanger; a liquid refrigerant pipe connected to each of the heatsource-side heat exchanger and the utilization-side heat exchanger; a low-pressure refrigerant pipe connected to each of the utilization-side heat exchanger and a suction port of the compressor; a liquid-side on-off valve disposed in the liquid refrigerant pipe; an expansion mechanism (410, 440) disposed in the liquid refrigerant pipe; a gas-side on-off valve disposed in the low-pressure refrigerant pipe; and an ambient temperature detector configured to detect temperature of the fluid which passes through the heatsource-side heat exchanger as ambient temperature, the controller being configured to control the heat pump system to perform a refrigerant recovery operation for recovering refrigerant from a utilization-side piping section to a heatsource-side piping section by operating the compressor while the liquid-side on-off valve is closed and the gas-side on-off valve is open, the utilization-side piping section extending between the liquid-side on-off valve and the gas-side on-off valve and including at least the utilization-side heat exchanger, the heatsource-side piping section extending between the gas-side on-off valve and the liquid-side on-off valve and including at least the compressor, characterized in that the controller is configured to, in the refrigerant recovery operation, control the compressor such that, when the ambient temperature is higher than or equal to a predetermined ambient temperature value, increase rate of compressor rotation speed is low compared with increase rate of compressor rotation speed of when the ambient temperature is lower than the predetermined ambient temperature value. the heat pump system comprising: a compressor; a heatsource-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a utilization-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a high-pressure refrigerant pipe connected to each of a discharge port of the compressor and the heatsource-side heat exchanger; a liquid refrigerant pipe connected to each of the heatsource-side heat exchanger and the utilization-side heat exchanger; a low-pressure refrigerant pipe connected to each of the utilization-side heat exchanger and a suction port of the compressor; a liquid-side on-off valve disposed in the liquid refrigerant pipe; an expansion mechanism (410, 440) disposed in the liquid refrigerant pipe; a gas-side on-off valve disposed in the low-pressure refrigerant pipe; and an ambient temperature detector configured to detect temperature of the fluid which passes through the heatsource-side heat exchanger as ambient temperature, a compressor; a heatsource-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a utilization-side heat exchanger configured to cause heat exchange between refrigerant flowing therein and fluid passing therethrough; a high-pressure refrigerant pipe connected to each of a discharge port of the compressor and the heatsource-side heat exchanger; a liquid refrigerant pipe connected to each of the heatsource-side heat exchanger and the utilization-side heat exchanger; a low-pressure refrigerant pipe connected to each of the utilization-side heat exchanger and a suction port of the compressor; a liquid-side on-off valve disposed in the liquid refrigerant pipe; an expansion mechanism (410, 440) disposed in the liquid refrigerant pipe; a gas-side on-off valve disposed in the low-pressure refrigerant pipe; and an ambient temperature detector configured to detect temperature of the fluid which passes through the heatsource-side heat exchanger as ambient temperature, the controller being configured to control the heat pump system to perform a refrigerant recovery operation for recovering refrigerant from a utilization-side piping section to a heatsource-side piping section by operating the compressor while the liquid-side on-off valve is closed and the gas-side on-off valve is open, the utilization-side piping section extending between the liquid-side on-off valve and the gas-side on-off valve and including at least the utilization-side heat exchanger, the heatsource-side piping section extending between the gas-side on-off valve and the liquid-side on-off valve and including at least the compressor, the utilization-side piping section extending between the liquid-side on-off valve and the gas-side on-off valve and including at least the utilization-side heat exchanger, the heatsource-side piping section extending between the gas-side on-off valve and the liquid-side on-off valve and including at least the compressor, characterized in that the controller is configured to, in the refrigerant recovery operation, control the compressor such that, when the ambient temperature is higher than or equal to a predetermined ambient temperature value, increase rate of compressor rotation speed is low compared with increase rate of compressor rotation speed of when the ambient temperature is lower than the predetermined ambient temperature value.