Abstract: In a cutting tool optimally used for working a valve hole in a cylinder head of an engine, a U-shaped groove (11) having a U-shaped cross section is formed to extend along a generatrix of a substantially conic tool main body (1), and a slider (160 carrying a cutting tip (5A) is slidably fitted in the U-shaped groove (11). A spacer (12) formed with a serration groove (12a) is detachably mounted between a wall surface (11a) of the U-shaped groove (11) and the slider (16). The slider (16) is formed on its side surface with a serration groove (16a) which engages the serration groove (12a). The slider (16) is biased toward the wall surface (11a) with an elastic force by a biasing means (15) which is provided at another wall surface (11b) of the U-shaped groove (11) and includes a pressing pin (17), a disk spring (18) and a set screw (19).