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). Therefore, abrasion and deformation can be easily coped with by replacing the spacer (12), and the shakiness of the slider (16) can be suppressed, thereby extending the life of the tool and reducing the labor for maintenance.