To provide a rotational power transmission device and a machine tool capable of further enhancing a rotating speed (capable of further increasing the rotating speed) of a main spindle, by transmitting large power without enlarging a diameter of the main spindle.

The rotational power transmission device has the main spindle 32, an auxiliary spindle 42 and a motor 24 provided with a driving roller 24R. A belt DB is extended to the main spindle and the auxiliary spindle arranged in parallel. A part of an unextended area of the belt of the main spindle is arranged in a main power transmission area 32A abutting on a cylindrical surface of the driving roller, and a part of an unextended area of the belt of the auxiliary spindle is set in an auxiliary power transmission area 42A abutting on the cylindrical surface of the driving roller. The main power transmission area and the auxiliary power transmission area are set in an opposed position. The driving roller is arranged so as to be pinched between the main power transmission area and the auxiliary power transmission area, and rotates the main spindle and the auxiliary spindle by frictional contact.