To cut a cable with straightness kept, by providing a cutting bite for cutting an insulating layer portion, between cutting grooves formed on an insulating layer, by a push down cutting blade.

Protruded quantity, to the frame body 10 of a screw spindle 13, is adjusted to push the pushdown cutting blades 19 and 19 of a cutter 18 to an insulating layer 2 to hold the grips 15 and 15, thereby rotating the body 10 on the layer 2 while rollingly moving respective two pairs of guide rollers 21 on a clamp body. This forms two treads of parallel cutting grooves by the push down cutting blades 19 and 19 on the peripheral surface of the layer 2. Then the protruded quantity of a screw spindle 14 is adjusted to bite the blade edge of a cutting bite 20 into the layer 2 between cutting grooves 2A and 2A. In this condition, the body 10 is rotatingly operated to cut part of the peripheral surface of the layer 2 portion by the cutting bite 20.