An external combustion engine provided with a pipe-shaped main container with a heating portion and a cooling portion, an output part converting displacement of the liquid part of said working fluid generated due to the change in volume of said working fluid accompanying generation and condensation of said steam to mechanical energy for output, a venturi provided at a communicating part of the main container and an auxiliary container, a communicating member forming a communicating passage bypassing the venturi and communicating the main container and the auxiliary container, and a shutting means for closing the communicating passage at the time of normal operation and opening the communicating passage at the time of startup, wherein at the time of startup, the pressure loss at the communicating passage becomes smaller than a saturated steam pressure at the temperature of the heating portion.