A cutting insert comprising first and second end surfaces (2, 3), a peripheral side surface extending between the first and second end surfaces (2, 3), and a cutting edge formed at least in part of an intersecting edge between the first end surface and the peripheral side surface, wherein: the cutting insert comprises: the cutting edge which includes at least one circular-arc-shaped cutting edge; and a contact surface portion formed in the second end surface; when a reference axis is defined as a line passing through a center of the first end surface and being perpendicular to the first end surface, and when a virtual circle is defined, as viewed from a direction facing the first end surface, so that the virtual circle includes, as its arc, a first circular-arc-shaped cutting edge (5a1) which is one of the circular-arc-shaped cutting edges, a center of the virtual circle is apart from the reference axis; and when a first reference plane is defined as a plane including the reference axis and extending through the center of the virtual circle; a second reference plane is defined as a plane extending perpendicularly to the reference axis and arranged on the first end surface side with respect to the second end surface; and a virtual plane is defined as a plane orthogonal to both the first reference plane and the second reference plane and extending so as to intersect with the first circular-arc-shaped cutting edge (5a1) and traverse the contact surface portion, characterised in that, the contact surface portion has, in the virtual plane, a portion which is inclined so as to gradually become more distant from the second reference plane heading from one end to the other, wherein in a side view in which the cutting edge which includes the first circular-arc-shaped cutting edge (5a1) is viewed from a direction facing the virtual plane, the cutting edge has an inclined portion which is inclined so as to form an acute angle with respect to the second reference plane; and the direction of inclination of the contact surface portion in the virtual plane is the same as the direction of inclination which is obtained when the shape of the inclined portion of the cutting edge in the side view is projected onto the virtual plane, wherein the contact surface portion is inclined in the virtual plane so as to make an acute angle with respect to the second reference plane, and an angle of the inclination (θ) is within a range of 3 degrees or more and 15 degrees or less. the cutting insert comprises: the cutting edge which includes at least one circular-arc-shaped cutting edge; and a contact surface portion formed in the second end surface; when a reference axis is defined as a line passing through a center of the first end surface and being perpendicular to the first end surface, and when a virtual circle is defined, as viewed from a direction facing the first end surface, so that the virtual circle includes, as its arc, a first circular-arc-shaped cutting edge (5a1) which is one of the circular-arc-shaped cutting edges, a center of the virtual circle is apart from the reference axis; and when a first reference plane is defined as a plane including the reference axis and extending through the center of the virtual circle; a second reference plane is defined as a plane extending perpendicularly to the reference axis and arranged on the first end surface side with respect to the second end surface; and a virtual plane is defined as a plane orthogonal to both the first reference plane and the second reference plane and extending so as to intersect with the first circular-arc-shaped cutting edge (5a1) and traverse the contact surface portion, characterised in that, the contact surface portion has, in the virtual plane, a portion which is inclined so as to gradually become more distant from the second reference plane heading from one end to the other, wherein in a side view in which the cutting edge which includes the first circular-arc-shaped cutting edge (5a1) is viewed from a direction facing the virtual plane, the cutting edge has an inclined portion which is inclined so as to form an acute angle with respect to the second reference plane; and the direction of inclination of the contact surface portion in the virtual plane is the same as the direction of inclination which is obtained when the shape of the inclined portion of the cutting edge in the side view is projected onto the virtual plane, wherein the contact surface portion is inclined in the virtual plane so as to make an acute angle with respect to the second reference plane, and an angle of the inclination (θ) is within a range of 3 degrees or more and 15 degrees or less.