There is provided a Rankine cycle system which generates electricity by using heat as an energy source, in which a conventional pump for carrying a working fluid is not used, thereby reducing the cost of manufacturing the Rankine cycle system, saving the electric power consumed to operate the pump and therefore obtaining a greater output of power, and in which a control system is used to automatically operate the Rankine cycle system, thereby enabling to stably operate the Rankine cycle system even though a heat source is intermittently supplied.