A method for converting thermal energy into mechanical energy by use of an organic rankine cycle system accommodating a working fluid composition, the method comprising sequentially performing: supplying the working fluid composition to an organic rankine cycle device that uses, used, or is designed to use a working fluid containing 1,1,1,3,3-pentafluoropropane as a main component or replacing the working fluid of the organic rankine cycle device that uses, used, or is designed to use a working fluid containing 1,1,1,3,3-pentafluoropropane as a main component with the working fluid composition; evaporating the working fluid composition at a temperature higher than or equal to 60°C and lower than or equal to 150°C; expanding the working fluid composition to convert thermal energy into mechanical energy; condensing the working fluid composition; and transferring the working fluid composition, while raising a pressure thereof, by a pump; wherein thermal energy of a temperature higher than or equal to 60°C and lower than or equal to 150°C is converted into mechanical energy by the method, wherein the working fluid composition contains cis-1,3,3,3-tetrafluoropropene at a ratio higher than or equal to 90.0% by mass and lower than or equal to 99.9% by mass and 1,1,1,3,3-pentafluoropropane at a ratio higher than or equal to 0.1% by mass and lower than or equal to 10.0% by mass, wherein the working fluid composition has a relative value of a power generation cycle efficiency (η cycle) higher than 1 where a corresponding value of 1,1,1,3,3-pentafluoropropane is 1 and a relative value of an expansion device size parameter less than 1 where a corresponding value of 1,1,1,3,3-pentafluoropropane is 1 when the working fluid composition is evaporated at a temperature higher than or equal to 60°C and lower than or equal to 150°C, and wherein a global warming potential is less than 150. supplying the working fluid composition to an organic rankine cycle device that uses, used, or is designed to use a working fluid containing 1,1,1,3,3-pentafluoropropane as a main component or replacing the working fluid of the organic rankine cycle device that uses, used, or is designed to use a working fluid containing 1,1,1,3,3-pentafluoropropane as a main component with the working fluid composition; evaporating the working fluid composition at a temperature higher than or equal to 60°C and lower than or equal to 150°C; expanding the working fluid composition to convert thermal energy into mechanical energy; condensing the working fluid composition; and transferring the working fluid composition, while raising a pressure thereof, by a pump; wherein thermal energy of a temperature higher than or equal to 60°C and lower than or equal to 150°C is converted into mechanical energy by the method, wherein the working fluid composition contains cis-1,3,3,3-tetrafluoropropene at a ratio higher than or equal to 90.0% by mass and lower than or equal to 99.9% by mass and 1,1,1,3,3-pentafluoropropane at a ratio higher than or equal to 0.1% by mass and lower than or equal to 10.0% by mass, wherein the working fluid composition has a relative value of a power generation cycle efficiency (η cycle) higher than 1 where a corresponding value of 1,1,1,3,3-pentafluoropropane is 1 and a relative value of an expansion device size parameter less than 1 where a corresponding value of 1,1,1,3,3-pentafluoropropane is 1 when the working fluid composition is evaporated at a temperature higher than or equal to 60°C and lower than or equal to 150°C, and wherein a global warming potential is less than 150.