To provide a cutting tool capable of stably performing cutting of a planar square shape such as a square hole having a corner part of a square shape for even a hard material to be cut such as steel.

This cutting tool 100 is mounted in a cutting device X including a mechanism for converting rotational movement of a rotary shaft into square shaft shape movement by at least two kinds of cams. The cutting tool 100 includes three bottom blades 103 at a distal end of a cutting portion 102 extended from a tool holding portion 101 of a substantially right triangular column shape. An outer peripheral distal end of each bottom blade is located on the same circumference. A line for connecting an axial center O and the outer peripheral distal end of each bottom blade and a line for connecting the axial center O and each apex angle portion of the tool holding portion 101 are made to coincide with each other on a plane. An outer peripheral relief angle θ of an outer peripheral blade is made more than 45° and 60° or below, and an outer rake angle α is made -10°or more and -40°or below. In the outer peripheral blade, a margin M of 0.02 mm or more is provided.