A method and system of converting low-temperature energy into high-temperature energy comprises steps of concentrating a high-boiling liquid diluted with a low-boiling liquid through evaporation of the low-boiling liquid; condensing the vapor of the low-boiling liquid through its absorption into an intermediate heat medium liquid, while cooling, evaporating the diluted intermediate heat medium, and absorbing the vapor generated in the last step into the concentrated high-boiling liquid thereby generating dilution heat and phase-change heat. The intermediate heat medium has the property that the difference between the saturation temperature thereof and the saturation temperature of the high-boiling liquid becomes small under a low pressure and the difference between the saturation temperatures of both the liquids becomes large under a high pressure.