PURPOSE: To no provide a transmission for a helicopter which is designed to reduce the generation of vibration and noise.

CONSTITUTION: The rotation force of a drive wheel l is transmitted to a driven wheel 2 through a traction to rotate the driven wheel 2. The rotation force of the driven wheel 2 produces an output which is transmitted. To hold a contact bearing between the drive wheel 1 and the driven wheel 2 at a value necessary enough to perform transmission of a power by means of a traction, an axial tension is generated at a ring 4, fixed to the drive wheel 1, through a carbon seal 5 by means of a bellows 6. This constitution keeps a contact bearing between the driven wheel 1 and the driven wheel 2 at a constant value.