A plate member of a cutting tool has two side surfaces being substantially rectangular in shape, a top surface and a bottom surface extending in a direction along a long side of the rectangle, and two end surfaces in a short-side direction. A cutting edge is located on the side of one of the end surfaces. One of the side surfaces is a curved surface bulging outward. The bottom surface has an inclined portion which is inclined with respect to the top surface. The plate member is held by a tool block having a bottom restraining surface which comes into contact with the bottom surface, and is fastened to the tool block by using a clamp member having a top restraining surface which comes into contact with the top surface of the plate member. This allows adjustment of a projection length of a cutting edge while holding the plate member in a stable manner, thereby suppressing chatter vibration and improving the quality of a finished surface.