PURPOSE: To open and close the intake port of a cylinder to perform two-cycle operation and four-cycle operation of an engine respectively without difficulties by providing a radial magnetic field and a vertical electric current to a seal valve wound over the outer periphery of the intake port portion at the lower part of the cylinder.

CONSTITUTION: An intake port opening and closing means 5 for supplying electric power to a fixed electrode 3, turning a seal valve 2 by electromagnetic force acting between a magnetic flux and an energized electric current, thereby opening and closing an intake port 13, is provided to make the operating cycle of an engine two-cycle by operation of an exhaust valve 8 and opening of the inlet port 13 when the engine is operated with high load at a low speed, and to make the operating cycle four-cycle by operation of an intake valve 7 and the exhaust valve 8 when the engine is operated with low load at a low speed or at a high speed.