Provided are: a working medium for a heat cycle, said working medium containing 1,2-difluoroethylene and exhibiting high stability, reduced environmental impact, and cycle performance that can replace that of R410A (a pseudo-azeotropic mixed refrigerant including difluoromethane and pentafluoroethane at a mass ratio of 1:1) and 1,1,1,2-tetrafluoroethane; a composition for heat cycle system, said composition using the medium; and a heat cycle system using the composition. The working medium for a heat cycle is characterized by the inclusion of 1,2-difluoroethylene and at least two types of hydrofluorocarbon selected from the group of hydrofluorocarbons that include a carbon-carbon double bond other than a saturated hydrofluorocarbon and 1,2-difluoroethylene.