The invention relates to a method for converting heat energy into mechanical energy by means of a Rankine cycle. In the Rankine cycle the circulating working fluid is pumped to a pressure above its critical pressure prior to heat exchange with an external medium. During the heat exchange with the external medium, the working fluid is heated to a temperature above its critical temperature and sufficiently high for the working fluid to expand without partial condensation. The working fluid is then expanded and condensed. The maximum pressure of the working fluid is controlled by means of an expander controllable with regard to the mass flow rate of the working fluid and/or a pump controllable with regard to the mass flow rate of the working fluid.