To provide a rotary engine, which converts constant velocity rotation of a rotary driving shaft to non-constant velocity rotation to transmit it to two pairs of rotors structured on the same shaft and which uses change of volume between pistons fixed to the rotors.

The angular velocity modulation rotation transmission mechanism is provided with the rotatable rotary driving shaft 3c, a tubular second rotor 2 having a pair of sector pistons 2a and 2b and a first rotor 1 having a pair of sector pistons 1a and 1b rotatably inserted into the shaft of the second rotor 2 inside a housing case 5. Planetary bevel gears 6 and 12 rotatably fitted to the periphery of the rotor 3 integrated with the rotary driving shaft 3c are engaged with a fixed bevel gear 4 having twice number of tooth, and the constant velocity rotation of the rotary driving shaft is modulated in angular velocity to convert to non-constant velocity rotation with swinging operation of the planetary bevel gear, and transmitted to the the first rotor and the second rotor.