To engage and interlock both smoothly and for a long time in spite of generating manufacturing errors and assembling installation errors at their bearing portions, while using a worm and a worm wheel having a contacted deceleration mechanism and a high reversion preventing effect for an axis for feeding threads, and to improve a manufacturing cost and the assembling installation.

A mounting portion K provided on a cutting rotor in a cutting unit A is made by being divided into a first mounting portion 6 bearing the axis 8 for feeding threads provided with the worm wheel 41 and a second mounting portion 4 bearing the axis 11 of rotational transmission provided with a passive rotor 9 and the worm 42, an adjustment permitting a relative movement in the coupling plane direction of both the mounting portions 6, 4 within the constant range in the first fixing means 17 fixing and coupling both the mounting portions 6, 4, and the second mounting means 18 fixing and coupling both the mounting portions 6, 4 are provided while maintaining the relatively positional relationship of both the mounting portions 6, 4 fixed and coupled with the first fixing means 17.