A refrigerant system, comprising: an air conditioner including: an outdoor heat exchanger configured to perform heat exchange between external air and a first refrigerant; an indoor heat exchanger configured to perform heat exchange between internal air and the first refrigerant; a first compressor for compressing the first refrigerant; a first refrigerant passageway of a first refrigerant heat exchanger; and a first expander configured to expand the first refrigerant at an intake side of the first refrigerant passageway; a cooler (2, 3) including: a refrigerator outdoor heat exchanger configured to perform heat exchange between external air and a second refrigerant; a refrigerator heat exchanger configured to perform heat exchange between internal air and the second refrigerant; a second compressor for compressing the second refrigerant and including a main compressor and an auxiliary compressor configured to backup the main compressor; and a second refrigerant passageway of the first refrigerant heat exchanger, wherein the cooler (2, 3) further includes: a third refrigerant passageway of a second refrigerant heat exchanger; a second expander configured to expand the second refrigerant at an intake side of the third passageway; a freezer outdoor heat exchanger configured to perform heat exchange between external air and a third refrigerant; a freezer heat exchanger configured to perform heat exchange between internal air and the third refrigerant; a third compressor for compressing the third refrigerant; and a fourth refrigerant passageway of the second refrigerant heat exchanger, wherein the first refrigerant heat exchanger is configured to exchange heat between the first refrigerant and the second refrigerant and the second refrigerant heat exchanger is configured to exchange heat between the second refrigerant and the third refrigerant. an air conditioner including: an outdoor heat exchanger configured to perform heat exchange between external air and a first refrigerant; an indoor heat exchanger configured to perform heat exchange between internal air and the first refrigerant; a first compressor for compressing the first refrigerant; a first refrigerant passageway of a first refrigerant heat exchanger; and a first expander configured to expand the first refrigerant at an intake side of the first refrigerant passageway; a cooler (2, 3) including: a refrigerator outdoor heat exchanger configured to perform heat exchange between external air and a second refrigerant; a refrigerator heat exchanger configured to perform heat exchange between internal air and the second refrigerant; a second compressor for compressing the second refrigerant and including a main compressor and an auxiliary compressor configured to backup the main compressor; and a second refrigerant passageway of the first refrigerant heat exchanger, wherein the cooler (2, 3) further includes: a third refrigerant passageway of a second refrigerant heat exchanger; a second expander configured to expand the second refrigerant at an intake side of the third passageway; a freezer outdoor heat exchanger configured to perform heat exchange between external air and a third refrigerant; a freezer heat exchanger configured to perform heat exchange between internal air and the third refrigerant; a third compressor for compressing the third refrigerant; and a fourth refrigerant passageway of the second refrigerant heat exchanger, wherein the first refrigerant heat exchanger is configured to exchange heat between the first refrigerant and the second refrigerant and the second refrigerant heat exchanger is configured to exchange heat between the second refrigerant and the third refrigerant. a third refrigerant passageway of a second refrigerant heat exchanger; a second expander configured to expand the second refrigerant at an intake side of the third passageway; a freezer outdoor heat exchanger configured to perform heat exchange between external air and a third refrigerant; a freezer heat exchanger configured to perform heat exchange between internal air and the third refrigerant; a third compressor for compressing the third refrigerant; and a fourth refrigerant passageway of the second refrigerant heat exchanger, wherein the first refrigerant heat exchanger is configured to exchange heat between the first refrigerant and the second refrigerant and the second refrigerant heat exchanger is configured to exchange heat between the second refrigerant and the third refrigerant.