A cutting tool in which a disc-like cutter blade is fixed to a spindle of a rotary electric tool, comprising a first guide plate, which is attached on electric tool side to a cutter blade such that it is capable of rotating freely and makes contact with the reference surface of a cutting object material on a process of cutting progress so as to guide along a cutting direction, an inner flange located between the first guide plate and the cutter blade so as to specify a distance X from the reference surface of the cutting object material to a cut groove and a second guide plate which is provided on an opposite side to the electric tool side of the cutter blade and makes contact with a cut face of the cutting object material on a process of the cutting progress so as to stop moving in the cutting direction to specify a cut groove depth Y.