In an attempt to regenerate energy of exhaust gas by using a pressure regenerator secured to an exhaust port of a displacement type engine so as to realize the Atkinson's cycle, if the engine is a four-cycle engine, raising regenerative pressure may cause abnormal fuel combustion or melting damage of an exhaust valve. The present invention has successfully solved this problem by using a six-cycle engine. The six-cycle engine with a regenerator according to the present invention is capable of varying proportion of the power output that is generated by the six-cycle engine itself and that is generated by the regenerator. Using this principle enables the six-cycle engine with a generator to be used as an engine that is capable of generating two kinds of output power or as a six-cycle gas turbine with high controllability. The invented internal combustion engine is excellent and valuable in the fuel cost economy when it is used as a motor for a power station and a small generator as well as a motor for a movable body such as a ship or a vehicle. The invented internal combustion engine is especially suitable for a movable body such as a hybrid-system vehicle. Incidentally, the inventor is ready for granting the rights for practicing the present invention.