A cutting insert (1; 21) attachable to a tool main body of a removable insert-type cutting tool, the cutting insert (1; 21) comprising: an insert main body having an upper surface, a lower surface and a plurality of side surfaces (4, 5, 6); and a first cutting edge formed at a ridge part between a first side surface as a rake face among the plurality of side surfaces and the upper surface, wherein the lower surface is formed in a flat surface shape which can be closely attached to the tool main body, the insert main body is provided with an attachment hole which penetrates the insert main body between the upper and lower surfaces (3, 3), a second side surface which is positioned on the opposite side of the insert main body with respect to the first side surface and given as a seating surface in contact with an insert seat of the tool main body for attaching the cutting insert, wherein the second side surface is formed in a V-shape when the insert main body is viewed in the thickness direction of the insert main body, in which the attachment hole penetrates the insert main body characterized in that the second side surface is formed in a recessed V-shape at a cross section along the thickness direction of the insert main body intersecting with the lower surface and the second side surface, of a first contact surface and a second contact surface assuming the recessed V-shape, on the first contact surface closer to the upper surface than the lower surface, there is formed a first contact portion which can be in contact with the tool main body, and the first contact portion is formed in such a manner that the first contact surface is raised in a curve shape at the cross section of the insert main body. an insert main body having an upper surface, a lower surface and a plurality of side surfaces (4, 5, 6); and a first cutting edge formed at a ridge part between a first side surface as a rake face among the plurality of side surfaces and the upper surface, wherein the lower surface is formed in a flat surface shape which can be closely attached to the tool main body, the insert main body is provided with an attachment hole which penetrates the insert main body between the upper and lower surfaces (3, 3), a second side surface which is positioned on the opposite side of the insert main body with respect to the first side surface and given as a seating surface in contact with an insert seat of the tool main body for attaching the cutting insert, wherein the second side surface is formed in a V-shape when the insert main body is viewed in the thickness direction of the insert main body, in which the attachment hole penetrates the insert main body characterized in that the second side surface is formed in a recessed V-shape at a cross section along the thickness direction of the insert main body intersecting with the lower surface and the second side surface, of a first contact surface and a second contact surface assuming the recessed V-shape, on the first contact surface closer to the upper surface than the lower surface, there is formed a first contact portion which can be in contact with the tool main body, and the first contact portion is formed in such a manner that the first contact surface is raised in a curve shape at the cross section of the insert main body.