To provide a cutting device for a straight member, which is composed so as to prevent scattering of the dust generated by cutting to an exterior by attaching a dust preventing means for easily adaptable to different outside dimensions, in regard to a cutting device rotating a rotary cutter at a high speed to cut straight members such wiring ducts with a plurality of sizes.

The cutting device for a straight member is provided with: the rotary cutter 2 for cutting the straight member W such as a duct; and a work mounting table for mounting the straight member and freely slidable in a direction approaching and separating away from the rotary cutter. It is composed such that a cutting part 3 provided with an insertion opening 32 for inserting the straight member is provided on the work mounting table, and that dust preventing means 4 having a gap matching with a shape of the straight member and a fluffy brush 42 covering up the gap and of a degree allowing insertion of the straight member, are provided on both sides of an outlet part and an inlet part of the insertion opening to prevent scattering of dust generated when carrying out cutting.