To obtain rotary engine making a rotary piston for making an internal surface of a cylinder as an outside surface perform simple rotary motion, sliding and contacting the internal surface of the cylinder and the outside surface of the rotary piston, at the inside of the simple cylinder making the internal surface of the cylinder as a simple circular surface.

At the inside of a cylinder 8 for making the internal surface of a cylinder as a circular surface, a rotary piston performing simple rotary motion is made into an A rotary piston 4 and a B rotary piston 5 assembled with hinges of four-leaf types. When the A rotary piston 4 and the B rotary piston 5 are alternately made to perform rotary motion repeating increasing and decreasing rotational speed, space held by the A rotary piston 4 and the B rotary piston 5 at the inside of the cylinder 8 becomes rotary motion repeating enlargement/contraction. Space surrounded by the A rotary piston 4 and the B rotary piston 5 is made a combustion chamber and rotary motion by repeating enlargement/contraction is applied to suction/compression/explosion/ exhaust operations.