An external combustion engine provided with a main container in which a working fluid is sealed flowable in a liquid phase state, a heater heating part of the liquid phase state working fluid in the main container to make it vaporize, a cooler cooling steam of the working fluid heated and vaporized by the heater so as to make it liquefy, an output part converting displacement of the liquid part of the working fluid caused by a change of volume of the steam into mechanical energy and outputting the energy, an auxiliary container communicated with the main container through a venturi means and having a liquid sealed inside it, an auxiliary heater heating the liquid in the auxiliary container to make it vaporize, a storage container communicated with the auxiliary container and storing the liquid, and a liquid draining means for draining liquid in the auxiliary container into the storage container when the internal pressure of the auxiliary container becomes a first predetermined pressure or more.