A method for producing a working fluid containing trifluoroethylene in a proportion of at most 70 mol% based on the entire amount, which comprises mixing a first component containing trifluoroethylene in a proportion of higher than 70 mol% stored in a first container, and a second component having no self-decomposition property containing at least one member selected from a saturated hydrofluorocarbon, an unsaturated hydrofluorocarbon, an unsaturated hydrochlorofluorocarbon and an unsaturated chlorofluorocarbon in a proportion of at least 30 mol%, stored in a second container, by the following step, or, wherein the first component is stored in the first container in a state where its temperature (t) and pressure (p) satisfy p[MPaG]<1.05-0.0025t at a temperature of at most 180°C: the first component is supplied to the second container under the following condition: the temperature of the first component at the time of the supply is t1<0.5°C, and at least during the period from the start of the supply to the completion of mixing, the relation between the gauge pressure in the second container and the gauge pressure of the first component at the time of the supply is kept to be p1>P2, and the temperature in the second container is kept to be in a state of T2<0.5°C; the second component is supplied to the first container under the following conditions and: at least during the period from the start of the supply to the completion of mixing, the temperature in the first container is kept to be in a state of T1<0.5°C; the gauge pressure of the second component is p2>P1 at least at the time of the supply, wherein P1 is the gauge pressure in the first container; to a separate third container, while at least during the period from the start of the following supply to the completion of mixing, the temperature in the third container is kept to be in a state of T3<0.5°C, the first component is supplied under the following condition, and the second component is supplied while at least at the time of the supply, its gauge pressure is kept to be p2>P3, wherein P3 is the gauge pressure in the third container: the temperature of the first component at the time of the supply is t1<0.5°C, and at least during the period from the start of the supply to the completion of mixing, the relation between the gauge pressure in the third container and the gauge pressure of the first component at the time of the supply is kept to be p1>P3, and the temperature in the third container is kept to be in a state of T3<0.5°C, wherein the first component and the second component are mixed in each of the steps, and in a state where they are both liquid. the first component is supplied to the second container under the following condition: the temperature of the first component at the time of the supply is t1<0.5°C, and at least during the period from the start of the supply to the completion of mixing, the relation between the gauge pressure in the second container and the gauge pressure of the first component at the time of the supply is kept to be p1>P2, and the temperature in the second container is kept to be in a state of T2<0.5°C; the temperature of the first component at the time of the supply is t1<0.5°C, and at least during the period from the start of the supply to the completion of mixing, the relation between the gauge pressure in the second container and the gauge pressure of the first component at the time of the supply is kept to be p1>P2, and the temperature in the second container is kept to be in a state of T2<0.5°C; the second component is supplied to the first container under the following conditions and: at least during the period from the start of the supply to the completion of mixing, the temperature in the first container is kept to be in a state of T1<0.5°C; the gauge pressure of the second component is p2>P1 at least at the time of the supply, wherein P1 is the gauge pressure in the first container; at least during the period from the start of the supply to the completion of mixing, the temperature in the first container is kept to be in a state of T1<0.5°C; the gauge pressure of the second component is p2>P1 at least at the time of the supply, wherein P1 is the gauge pressure in the first container; to a separate third container, while at least during the period from the start of the following supply to the completion of mixing, the temperature in the third container is kept to be in a state of T3<0.5°C, the first component is supplied under the following condition, and the second component is supplied while at least at the time of the supply, its gauge pressure is kept to be p2>P3, wherein P3 is the gauge pressure in the third container: the temperature of the first component at the time of the supply is t1<0.5°C, and at least during the period from the start of the supply to the completion of mixing, the relation between the gauge pressure in the third container and the gauge pressure of the first component at the time of the supply is kept to be p1>P3, and the temperature in the third container is kept to be in a state of T3<0.5°C,