An air conditioner comprising a fin-tube type heat exchanger comprising: a heat exchange section and headers, wherein the heat exchange section is a part to exchange heat between a refrigerant and air and wherein the heat exchange section has a plurality of heat-transfer fins and a plurality of heat-transfer pipes, the heat-transfer pipes each have opposite ends that communicate with respective headers, which are a distribution/collection header, which distributes/collects refrigerant to/from the heat-transfer pipes, and a fold back header, that bundle the heat-transfer pipes, an interior of the fold back header is divided by a plurality of first partition plates into compartments each of which is assigned to respective one of inflow paths and outflow paths, an interior of the distribution section is divided by a plurality of second partition plates into compartments each of which is assigned to respective one of the inflow paths in a similar manner to the fold back header, an interior of the collection section is divided by the plurality of second partition plates into compartments each of which is assigned to respective one of the outflow paths in a similar manner to the fold back header, the plurality of heat-transfer pipes arranged to extend in a horizontal direction and to be spaced apart at predetermined intervals in a vertical direction and configured to allow a thermal medium to flow therein, wherein a part of the plurality of heat transfer pipes are used for at least one inflow path into which the thermal medium flows from an outside of the heat exchanger and the other part of the plurality of heat transfer pipes are used for at least one outflow path from which the thermal medium flows out to the outside of the heat exchanger; and connection pipes through each of which an outlet side of a corresponding one or corresponding ones of the at least one inflow path communicates with an inlet side of a corresponding one or corresponding ones of the at least one outflow path, the connection pipes are constituted by at least one down-flow pipe and at least one up-flow pipe, the connection pipes having a hydraulic diameter of 4 mm or greater, wherein a circulation flow rate Gr kg/s of the thermal medium and a number of the connection pipes N satisfy 0.003 ≤ Gr/N ≤ 0.035, and wherein the thermal medium comprises at least one of the group consisting of R410A, R404A, R32, R1234yf, R1234ze, and HFO1123. a heat exchange section and headers, wherein the heat exchange section is a part to exchange heat between a refrigerant and air and wherein the heat exchange section has a plurality of heat-transfer fins and a plurality of heat-transfer pipes, the heat-transfer pipes each have opposite ends that communicate with respective headers, which are a distribution/collection header, which distributes/collects refrigerant to/from the heat-transfer pipes, and a fold back header, that bundle the heat-transfer pipes, an interior of the fold back header is divided by a plurality of first partition plates into compartments each of which is assigned to respective one of inflow paths and outflow paths, an interior of the distribution section is divided by a plurality of second partition plates into compartments each of which is assigned to respective one of the inflow paths in a similar manner to the fold back header, an interior of the collection section is divided by the plurality of second partition plates into compartments each of which is assigned to respective one of the outflow paths in a similar manner to the fold back header, the plurality of heat-transfer pipes arranged to extend in a horizontal direction and to be spaced apart at predetermined intervals in a vertical direction and configured to allow a thermal medium to flow therein, wherein a part of the plurality of heat transfer pipes are used for at least one inflow path into which the thermal medium flows from an outside of the heat exchanger and the other part of the plurality of heat transfer pipes are used for at least one outflow path from which the thermal medium flows out to the outside of the heat exchanger; and connection pipes through each of which an outlet side of a corresponding one or corresponding ones of the at least one inflow path communicates with an inlet side of a corresponding one or corresponding ones of the at least one outflow path, the connection pipes are constituted by at least one down-flow pipe and at least one up-flow pipe, the connection pipes having a hydraulic diameter of 4 mm or greater, wherein a circulation flow rate Gr kg/s of the thermal medium and a number of the connection pipes N satisfy 0.003 ≤ Gr/N ≤ 0.035, and wherein the thermal medium comprises at least one of the group consisting of R410A, R404A, R32, R1234yf, R1234ze, and HFO1123.