A water purifying filter assembly installable in and removable from a refrigerator through a rectangular installation hole (10b-1) of the refrigerator, the water purifying filter assembly comprising: a filter; an inner housing formed in a circular tube shape, configured to accommodate the filter, and configured to guide water to the filter; and an outer housing configured to accommodate the inner housing, wherein the outer housing is formed in a rectangular tube shape; and one side of the outer housing is open to accommodate the inner housing, wherein, while the inner housing is accommodated in the outer housing, the inner housing and the outer housing are spaced apart from each other to form an insulation space configured to suppress heat transfer therebetween, wherein one sides, which correspond to each other, of the inner housing and the outer housing are open; and the water purifying filter assembly further includes a connecting cap configured to cover the open one sides of the inner housing and the outer housing and configured to be connected to a connection valve of the refrigerator to guide water introduced through the connection valve to the filter, wherein the outer housing includes a stopper formed in a shape corresponding to the rectangular installation hole (10b-1), anda body configured to accommodate the inner housing and having a rectangular shape corresponding to the rectangular installation hole (10b-1), and a rotary guide having a cylindrical shape, configured to guide rotation of the water purifying filter assembly and provided between the stopper and the body) and wherein the water purifying filter assembly is configured to rotate to be coupled to the connection valve when the rotary guide is located in the rectangular installation hole (10b-1) while the water purifying filter assembly is being inserted through the rectangular installation hole (10b-1). a filter; an inner housing formed in a circular tube shape, configured to accommodate the filter, and configured to guide water to the filter; and an outer housing configured to accommodate the inner housing, wherein the outer housing is formed in a rectangular tube shape; and one side of the outer housing is open to accommodate the inner housing, wherein, while the inner housing is accommodated in the outer housing, the inner housing and the outer housing are spaced apart from each other to form an insulation space configured to suppress heat transfer therebetween, wherein one sides, which correspond to each other, of the inner housing and the outer housing are open; and the water purifying filter assembly further includes a connecting cap configured to cover the open one sides of the inner housing and the outer housing and configured to be connected to a connection valve of the refrigerator to guide water introduced through the connection valve to the filter, wherein the outer housing includes a stopper formed in a shape corresponding to the rectangular installation hole (10b-1), anda body configured to accommodate the inner housing and having a rectangular shape corresponding to the rectangular installation hole (10b-1), and a rotary guide having a cylindrical shape, configured to guide rotation of the water purifying filter assembly and provided between the stopper and the body) and wherein the water purifying filter assembly is configured to rotate to be coupled to the connection valve when the rotary guide is located in the rectangular installation hole (10b-1) while the water purifying filter assembly is being inserted through the rectangular installation hole (10b-1).