In a body for a cutting tool having an insert seat, to which a cutting insert is removably attached, not only a structure for preventing a crack at the insert seat, caused by stress concentration, is adopted, but also the cutting insert is stably seated at and firmly attached to the insert seat. A recessed curve portion is formed at a crossing portion between a bottom surface and a wall surface of the insert seat. As viewed in cross section perpendicular to the bottom surface, the cross-sectional shape of the curve portion includes an arc having a radius R of curvature within a range from 0.5-1.2 mm. A height from the bottom surface to a connecting portion between the wall surface and the curve portion is set within a range from 0.1-0.8 mm.