To increase a cutting speed without increasing the rotating speed by connecting a motor to an input side of an internal planetary gear mechanism, and mounting a cutting blade at an output side.

A blade fixing table 47 is a ring gear 52 comprising cycloid teeth on its inner peripheral surface, and a gear case 6 comprises a gear part 51 comprising the cycloid teeth. An internal planetary gear mechanism 5 is formed by the gear part 51 and the ring gear 52. A motor 4 is connected to an input side of the internal planetary gear mechanism 5, and a circular saw blade 3 is mounted at an output side. The rotation of the motor 4 is decelerated and transmitted to a driving shaft 45. The circular saw blade 3 is rotated and driven by the rotation of the driving shaft 45, but the circular saw blade 3 is not directly fixed to the driving shaft 45, but fixed to the blade fixing table 47 reversably mounted on an outer periphery of a decentering shaft 46 mounted on the driving shaft 45. Thereby the circular saw blade 3 is provided with not only the simple rotation but also the sliding motion, and a cutting speed can be increased.