A refrigerant cycle system comprising: a primary-side cycle of a vapor compression type, the primary-side cycle circulating a first refrigerant; a secondary-side cycle of a vapor compression type, the secondary-side cycle circulating a second refrigerant; and a cascade heat exchanger that exchanges heat between the first refrigerant and the second refrigerant, wherein the primary-side cycle includes a heat-source heat exchanger for giving cold or heat to the first refrigerant, and a primary-side connection pipe (21, 22) that connects the cascade heat exchanger and the heat-source heat exchanger, the secondary-side cycle includes a usage heat exchanger for using the cold or the heat obtained by the second refrigerant from the cascade heat exchanger, and a secondary-side connection pipe (41, 42) that connects the cascade heat exchanger and the usage heat exchanger, the primary-side connection pipe includes a primary-side gas connection pipe and a primary-side liquid connection pipe, and the secondary-side connection pipe includes a secondary-side gas connection pipe and a secondary-side liquid connection pipe, characterized in that the first refrigerant and the second refrigerant are the same refrigerant, and in that (a) a pipe diameter of the secondary-side gas connection pipe is smaller than a pipe diameter of the primary-side gas connection pipe, or (b) a pipe diameter of the secondary-side liquid connection pipe is smaller than a pipe diameter of the primary-side liquid connection pipe. a primary-side cycle of a vapor compression type, the primary-side cycle circulating a first refrigerant; a secondary-side cycle of a vapor compression type, the secondary-side cycle circulating a second refrigerant; and a cascade heat exchanger that exchanges heat between the first refrigerant and the second refrigerant, wherein the primary-side cycle includes a heat-source heat exchanger for giving cold or heat to the first refrigerant, and a primary-side connection pipe (21, 22) that connects the cascade heat exchanger and the heat-source heat exchanger, the secondary-side cycle includes a usage heat exchanger for using the cold or the heat obtained by the second refrigerant from the cascade heat exchanger, and a secondary-side connection pipe (41, 42) that connects the cascade heat exchanger and the usage heat exchanger, the primary-side connection pipe includes a primary-side gas connection pipe and a primary-side liquid connection pipe, and the secondary-side connection pipe includes a secondary-side gas connection pipe and a secondary-side liquid connection pipe, characterized in that the first refrigerant and the second refrigerant are the same refrigerant, and in that (a) a pipe diameter of the secondary-side gas connection pipe is smaller than a pipe diameter of the primary-side gas connection pipe, or (b) a pipe diameter of the secondary-side liquid connection pipe is smaller than a pipe diameter of the primary-side liquid connection pipe. (a) a pipe diameter of the secondary-side gas connection pipe is smaller than a pipe diameter of the primary-side gas connection pipe, or (b) a pipe diameter of the secondary-side liquid connection pipe is smaller than a pipe diameter of the primary-side liquid connection pipe.