A working medium for a refrigeration cycle, the working medium comprising three components: 1,1,2-trifluoroethylene and difluoromethane as refrigerant components; and a haloalkane having 1 to 4 carbon atoms but excluding a case where one or more halogen atoms are all fluorine as a disproportionation inhibitor capable for suppressing a disproportionation reaction under higher than or equal to 150°C temperature conditions, when a ternary phase diagram according to triangular coordinates (A, B, C) represents mass-ratio composition of the three components: the 1,1,2-trifluoroethylene; the difluoromethane; and the haloalkane, the mass-ratio composition of the three components having a composition range that corresponds to a combined region formed of: a first overlapping region where a region a and a region b overlap one another, with the region a representing more than 0 mass% and less than or equal to 65 mass% of the 1,1,2-trifluoroethylene, and the region b representing more than 0 mass% and less than or equal to 30 mass% of the difluoromethane; and a second overlapping region where a region c and the region b overlap one another, with the region c representing more than or equal to 20 mass% and less than 100 mass% of the haloalkane, wherein the haloalkane is at least any selected from the group consisting of diiodomethane CH 2 I 2, difluorodiiodomethane CF 2 I 2, 1-bromo-2-iodotetrafluoroethane CF 2 BrCF 2 I, 1,1,1-trifluoro-2-iodoethane CF 3 (CH 2)I, 1,1,1-trifluoro-3-iodopropane CF 3 (CH 2) 2 I, and 1,1,1-trifluoro-3-iodobutane CF 3 (CH 2) 3 I, characterized in that the mass-ratio composition of the three components formed of the 1,1,2-trifluoroethylene, the difluoromethane, and the haloalkane is within a composition range corresponding to a region surrounded by: a line L1 representing 30 mass% of the difluoromethane; a line L3 connecting between a point p2 as (A, B, C) = (65, 15, 20) and a point p3 as (A, B, C) = (65, 30, 5) on the line L1; and a line L5 connecting between a point p6 as (A, B, C) = (50, 30, 20) on the line L1 and the point p2. 1,1,2-trifluoroethylene and difluoromethane as refrigerant components; and a haloalkane having 1 to 4 carbon atoms but excluding a case where one or more halogen atoms are all fluorine as a disproportionation inhibitor capable for suppressing a disproportionation reaction under higher than or equal to 150°C temperature conditions, when a ternary phase diagram according to triangular coordinates (A, B, C) represents mass-ratio composition of the three components: the 1,1,2-trifluoroethylene; the difluoromethane; and the haloalkane, the mass-ratio composition of the three components having a composition range that corresponds to a combined region formed of: a first overlapping region where a region a and a region b overlap one another, with the region a representing more than 0 mass% and less than or equal to 65 mass% of the 1,1,2-trifluoroethylene, and the region b representing more than 0 mass% and less than or equal to 30 mass% of the difluoromethane; and a second overlapping region where a region c and the region b overlap one another, with the region c representing more than or equal to 20 mass% and less than 100 mass% of the haloalkane, wherein the haloalkane is at least any selected from the group consisting of diiodomethane CH 2 I 2, difluorodiiodomethane CF 2 I 2, 1-bromo-2-iodotetrafluoroethane CF 2 BrCF 2 I, 1,1,1-trifluoro-2-iodoethane CF 3 (CH 2)I, 1,1,1-trifluoro-3-iodopropane CF 3 (CH 2) 2 I, and 1,1,1-trifluoro-3-iodobutane CF 3 (CH 2) 3 I, characterized in that the mass-ratio composition of the three components formed of the 1,1,2-trifluoroethylene, the difluoromethane, and the haloalkane is within a composition range corresponding to a region surrounded by: a line L1 representing 30 mass% of the difluoromethane; a line L3 connecting between a point p2 as (A, B, C) = (65, 15, 20) and a point p3 as (A, B, C) = (65, 30, 5) on the line L1; and a line L5 connecting between a point p6 as (A, B, C) = (50, 30, 20) on the line L1 and the point p2. a first overlapping region where a region a and a region b overlap one another, with the region a representing more than 0 mass% and less than or equal to 65 mass% of the 1,1,2-trifluoroethylene, and the region b representing more than 0 mass% and less than or equal to 30 mass% of the difluoromethane; and a second overlapping region where a region c and the region b overlap one another, with the region c representing more than or equal to 20 mass% and less than 100 mass% of the haloalkane, wherein the haloalkane is at least any selected from the group consisting of diiodomethane CH 2 I 2, difluorodiiodomethane CF 2 I 2, 1-bromo-2-iodotetrafluoroethane CF 2 BrCF 2 I, 1,1,1-trifluoro-2-iodoethane CF 3 (CH 2)I, 1,1,1-trifluoro-3-iodopropane CF 3 (CH 2) 2 I, and 1,1,1-trifluoro-3-iodobutane CF 3 (CH 2) 3 I, characterized in that the mass-ratio composition of the three components formed of the 1,1,2-trifluoroethylene, the difluoromethane, and the haloalkane is within a composition range corresponding to a region surrounded by: a line L1 representing 30 mass% of the difluoromethane; a line L3 connecting between a point p2 as (A, B, C) = (65, 15, 20) and a point p3 as (A, B, C) = (65, 30, 5) on the line L1; and a line L5 connecting between a point p6 as (A, B, C) = (50, 30, 20) on the line L1 and the point p2. a line L1 representing 30 mass% of the difluoromethane; a line L3 connecting between a point p2 as (A, B, C) = (65, 15, 20) and a point p3 as (A, B, C) = (65, 30, 5) on the line L1; and a line L5 connecting between a point p6 as (A, B, C) = (50, 30, 20) on the line L1 and the point p2.