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 ; 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 , 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 ; 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 , 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 . <EOS>