WORKING FLUID FOR HEAT CYCLE, COMPOSITION FOR HEAT CYCLE SYSTEM, AND HEAT CYCLE SYSTEM

To provide a working fluid which has cycle performance sufficient as an alternative to R410A while the influence over global warming is sufficiently suppressed, which does not significantly increase the load to an apparatus as compared with a case where R410A is used, and which can be stably used continuously without any special measures, a composition for a heat cycle system comprising the working fluid, and a heat cycle system employing the composition.A working fluid for heat cycle, wherein the global warming potential is less than 300; the product of the relative coefficient of performance and the relative refrigerating capacity is at least 0.820 relative to R410A in a standard refrigerating cycle under conditions of an evaporation temperature of 0°C, a condensing temperature of 40°C, a supercoiling degree of 5°C and a degree of superheat of 5°C; the relative compressor discharge gas pressure is at most 1.100; the lower limit of the combustion range by method A in High Pressure Gas Safety Act is at least 5 vol%; and the pressure will not exceed 2.00 MPaG in a combustion test by method A in High Pressure Gas Safety act under 0.98 MPaG at 250°C.