To provide a new working medium for heat cycles, which is nonflammable and has a slight load on the environment and further improved heat cycle characteristics.

The working medium for heat cycles comprises at least 50 mass% of at least one selected from the group consisting of 1,2-dichloro-3,3,3-trifluoropropene, 1,3-dichloro-3,3-difluoropropene, 2-chloro-3,3,3-trifluoropropene and cis-1-chloro-3,3,3-trifluoropropene.