A tip-replaceable cutting tool comprising: an insert attachment seat for removably attaching a cutting insert; and a substantially plate-shaped cutting insert installed in the insert attachment seat, wherein said insert attachment seat has a substantially flat bottom face, and first and second restraint wall faces (7a, 7b) which rise upward from said bottom face and are not in parallel with each other, an angle (α) between the first and second restraint wall faces (7a, 7b) is smaller than an angle (β) between two peripheral faces (17a, 17b) of the cutting insert respectively opposing the first and second restraint wall faces (7a, 7b), as viewed from a side opposing the bottom face, a lower face of said cutting insert contacts said bottom face and said two peripheral faces (17a, 17b) contact said first and second restraint wall faces (7a, 7b) respectively opposing the peripheral faces (17a, 17b), one of said two peripheral faces (17a, 17b) of the cutting insert contacts said first restraint wall face in a substantially parallel posture with an extending direction of said first restraint wall face, and the other peripheral face is inclined relative to an extending direction of said second restraint wall face, and contacts said second restraint wall face at one end portion of said second restraint wall face, characterized in that said cutting insert has an attachment hole formed to penetrate a central portion thereof in a thickness direction, and said insert attachment seat includes a female screw hole into which a fastening screw is threaded for attaching the cutting insert, wherein as viewed from a side opposing the bottom face of said insert attachment seat, the center point of the female screw hole is closer to the first and second restraint wall faces (7a, 7b) along a virtual straight line passing the center point of said attachment hole, passing the center point of hole of said female screw hole and being inclined at an inclination angle (ε) to a perpendicular line drawn from a center point of said attachment hole of the cutting insert placed on said insert attachment seat to said first restraint wall face, wherein said inclination angle (ε) is larger than 0° and less than 1/2 of the angle (α) between said first and second restraint wall faces (7a, 7b). an insert attachment seat for removably attaching a cutting insert; and a substantially plate-shaped cutting insert installed in the insert attachment seat, wherein said insert attachment seat has a substantially flat bottom face, and first and second restraint wall faces (7a, 7b) which rise upward from said bottom face and are not in parallel with each other, an angle (α) between the first and second restraint wall faces (7a, 7b) is smaller than an angle (β) between two peripheral faces (17a, 17b) of the cutting insert respectively opposing the first and second restraint wall faces (7a, 7b), as viewed from a side opposing the bottom face, a lower face of said cutting insert contacts said bottom face and said two peripheral faces (17a, 17b) contact said first and second restraint wall faces (7a, 7b) respectively opposing the peripheral faces (17a, 17b), one of said two peripheral faces (17a, 17b) of the cutting insert contacts said first restraint wall face in a substantially parallel posture with an extending direction of said first restraint wall face, and the other peripheral face is inclined relative to an extending direction of said second restraint wall face, and contacts said second restraint wall face at one end portion of said second restraint wall face, characterized in that said cutting insert has an attachment hole formed to penetrate a central portion thereof in a thickness direction, and said insert attachment seat includes a female screw hole into which a fastening screw is threaded for attaching the cutting insert, wherein as viewed from a side opposing the bottom face of said insert attachment seat, the center point of the female screw hole is closer to the first and second restraint wall faces (7a, 7b) along a virtual straight line passing the center point of said attachment hole, passing the center point of hole of said female screw hole and being inclined at an inclination angle (ε) to a perpendicular line drawn from a center point of said attachment hole of the cutting insert placed on said insert attachment seat to said first restraint wall face, wherein said inclination angle (ε) is larger than 0° and less than 1/2 of the angle (α) between said first and second restraint wall faces (7a, 7b).