Provided is a method for converting thermal energy into mechanical energy by use of an organic rankine cycle system that accommodates a working fluid composition. The method includes sequentially performing: evaporating the working fluid composition; expanding the working fluid composition; condensing the working fluid composition; and transferring the working fluid composition, while raising a pressure thereof, by a pump. The working fluid composition contains cis-1,3,3,3-tetrafluoropropene at a ratio higher than or equal to 92.0% by mass and lower than or equal to 99.9% by mass and trans-1,3,3,3-tetrafluoropropene or 2,3,3,3-tetrafluoropropene at a ratio higher than or equal to 0.1% by mass and lower than or equal to 8.0% by mass; and has an evaporation temperature higher than or equal to 60°C and lower than or equal to 150°C.