A heat pump, comprising: a coolant main circuit that includes a scroll compressor, a condenser that condenses coolant compressed by the compressor, an expander that expands coolant condensed by the condenser, and an evaporator that evaporates coolant expanded by the expander; a first coolant injection circuit that extends from a first point on the coolant main circuit between the condenser and the evaporator to a first point on the compressor between a coolant inlet and a coolant outlet thereof; a second coolant injection circuit that extends from a second point on the coolant main circuit between the condenser and the evaporator to a second point on the compressor between the coolant inlet and the coolant outlet thereof, wherein the first point on the compressor is placed closer to the outlet of the compressor than the second point on the compressor; a first internal heat exchanger that heat-exchanges the coolant flown through the first coolant injection circuit and the coolant flown through the coolant main circuit; a second internal heat exchanger that heat-exchanges the coolant flown through the second coolant injection circuit and the coolant flown through the coolant main circuit; a first expander installed at the first coolant injection circuit; a second expander installed at the second coolant injection circuit; and a controller that controls opening degrees of the first and second expanders (32, 34), wherein the controller is configured to selectively open and close the first and second coolant injection circuits (101a, 101b) to generate the respective preset middle pressures, wherein the controller is configured to de-activate the first coolant injection circuit or the second coolant injection circuit when a respective supercooling degree exceeds a preset supercooling degree corresponding to a condensation temperature of the coolant, wherein the controller de-activates a corresponding one of the first or second coolant injection circuits (101a, 101b) when a volume ratio of the first coolant injection circuit is less than the first preset volume ratio or a volume ratio of the second coolant injection circuit is greater than the second preset volume ratio, wherein a volume ratio of the condensed coolant and the evaporated coolant corresponding to a first middle pressure is a first preset volume ratio and a volume ratio of the condensed coolant and the evaporated coolant corresponding to a second middle pressure is a second preset volume ratio, wherein the first middle pressure is a pressure of the coolant expanded by the first expander, and the second middle pressure is a pressure of the coolant expanded by the second expander. a coolant main circuit that includes a scroll compressor, a condenser that condenses coolant compressed by the compressor, an expander that expands coolant condensed by the condenser, and an evaporator that evaporates coolant expanded by the expander; a first coolant injection circuit that extends from a first point on the coolant main circuit between the condenser and the evaporator to a first point on the compressor between a coolant inlet and a coolant outlet thereof; a second coolant injection circuit that extends from a second point on the coolant main circuit between the condenser and the evaporator to a second point on the compressor between the coolant inlet and the coolant outlet thereof, wherein the first point on the compressor is placed closer to the outlet of the compressor than the second point on the compressor; a first internal heat exchanger that heat-exchanges the coolant flown through the first coolant injection circuit and the coolant flown through the coolant main circuit; a second internal heat exchanger that heat-exchanges the coolant flown through the second coolant injection circuit and the coolant flown through the coolant main circuit; a first expander installed at the first coolant injection circuit; a second expander installed at the second coolant injection circuit; and a controller that controls opening degrees of the first and second expanders (32, 34), wherein the controller is configured to selectively open and close the first and second coolant injection circuits (101a, 101b) to generate the respective preset middle pressures, wherein the controller is configured to de-activate the first coolant injection circuit or the second coolant injection circuit when a respective supercooling degree exceeds a preset supercooling degree corresponding to a condensation temperature of the coolant, wherein the controller de-activates a corresponding one of the first or second coolant injection circuits (101a, 101b) when a volume ratio of the first coolant injection circuit is less than the first preset volume ratio or a volume ratio of the second coolant injection circuit is greater than the second preset volume ratio, wherein a volume ratio of the condensed coolant and the evaporated coolant corresponding to a first middle pressure is a first preset volume ratio and a volume ratio of the condensed coolant and the evaporated coolant corresponding to a second middle pressure is a second preset volume ratio, wherein the first middle pressure is a pressure of the coolant expanded by the first expander, and the second middle pressure is a pressure of the coolant expanded by the second expander.