A cascade refrigeration system, comprising: a higher-stage refrigeration cycle in which a higher-stage compressor, a higher-stage condenser, a higher-stage expansion valve, and a higher-stage evaporator are sequentially connected by pipes and in which a higher-stage refrigerant is circulated; a lower-stage refrigeration cycle in which a lower-stage compressor, a first lower-stage condenser, a second lower-stage condenser, a lower-stage liquid receiver, a first lower-stage expansion valve, and a lower-stage evaporator are sequentially connected by pipes and in which a lower-stage refrigerant is circulated; and a cascade condenser including the higher-stage evaporator and the second lower-stage condenser to exchange heat between the higher-stage refrigerant flowing in the higher-stage evaporator and the lower-stage refrigerant flowing in the second lower-stage condenser, wherein the lower-stage refrigeration cycle is provided with a natural circulation circuit having a vapor refrigerant pipe that connects the lower-stage liquid receiver and a position between the first lower-stage condenser and the second lower-stage condenser and that has a check valve at a position in a middle of the vapor refrigerant pipe, and a second lower-stage expansion valve being provided between the second lower-stage condenser and the lower-stage liquid receiver, and wherein the natural circulation circuit has a subcooled refrigerant pipe that connects a position between the second lower-stage condenser and the second lower-stage expansion valve and a position between the second lower-stage expansion valve and the lower-stage liquid receiver and that has a pressure adjustment unit at a position in a middle of the subcooled refrigerant pipe, and the vapor refrigerant pipe is connected at a position between the second lower-stage expansion valve and the lower-stage liquid receiver. a higher-stage refrigeration cycle in which a higher-stage compressor, a higher-stage condenser, a higher-stage expansion valve, and a higher-stage evaporator are sequentially connected by pipes and in which a higher-stage refrigerant is circulated; a lower-stage refrigeration cycle in which a lower-stage compressor, a first lower-stage condenser, a second lower-stage condenser, a lower-stage liquid receiver, a first lower-stage expansion valve, and a lower-stage evaporator are sequentially connected by pipes and in which a lower-stage refrigerant is circulated; and a cascade condenser including the higher-stage evaporator and the second lower-stage condenser to exchange heat between the higher-stage refrigerant flowing in the higher-stage evaporator and the lower-stage refrigerant flowing in the second lower-stage condenser, wherein the lower-stage refrigeration cycle is provided with a natural circulation circuit having a vapor refrigerant pipe that connects the lower-stage liquid receiver and a position between the first lower-stage condenser and the second lower-stage condenser and that has a check valve at a position in a middle of the vapor refrigerant pipe, and a second lower-stage expansion valve being provided between the second lower-stage condenser and the lower-stage liquid receiver, and wherein the natural circulation circuit has a subcooled refrigerant pipe that connects a position between the second lower-stage condenser and the second lower-stage expansion valve and a position between the second lower-stage expansion valve and the lower-stage liquid receiver and that has a pressure adjustment unit at a position in a middle of the subcooled refrigerant pipe, and the vapor refrigerant pipe is connected at a position between the second lower-stage expansion valve and the lower-stage liquid receiver.