A cutting insert to be detachably attached to an insert mounting seat formed at a tip end portion of an end mill main body of an indexable ball end mill rotatable about an axis, the cutting insert comprising: a rake face facing a direction of rotation of the end mill main body; a seating face facing an opposite side to the rake face and configured to be seated on a bottom surface of the insert mounting seat; and a flank face extending around the rake face and the seating face, wherein two cutting edges (5, 6) each having an arcuate cutting edge portion (5a, 6a) extending in an arc shape in plan view when viewed from a direction opposed to the rake face and a linear cutting edge portion (5b, 6b) extending so as to be in contact with the arcuate cutting edge portion (5a, 6a) are formed on an intersecting ridgeline portion between the rake face and the flank face so that the arcuate cutting edge portion (5a, 6a) and the linear cutting edge portion (5b, 6b) are alternately positioned in a circumferential direction of the rake face, a groove portion having a wall surface (8a, 8b) configured to be capable of coming into contact with a projection portion (14A, 14B, 14C) protruding from the bottom surface of the insert mounting seat is formed on the seating face, the groove portion has a narrow width portion in which a groove width decreases from one end side toward the other end side in a direction in which the groove portion extends, wherein one of the two cutting edges (5, 6) is a main cutting edge and the other is a minor cutting edge, the arcuate cutting edge portion of the minor cutting edge is formed to have a same radius as a radius of the arcuate cutting edge portion of the main cutting edge and have a shorter length than a length of the arcuate cutting edge portion of the main cutting edge in the circumferential direction, and the groove portion extends from an arcuate cutting edge portion side of the minor cutting edge toward a linear cutting edge portion side of the main cutting edge, characterized in that the linear cutting edge portion side of the main cutting edge is set as the one end side, and the arcuate cutting edge portion side of the minor cutting edge is set as the other end side. a rake face facing a direction of rotation of the end mill main body; a seating face facing an opposite side to the rake face and configured to be seated on a bottom surface of the insert mounting seat; and a flank face extending around the rake face and the seating face, wherein two cutting edges (5, 6) each having an arcuate cutting edge portion (5a, 6a) extending in an arc shape in plan view when viewed from a direction opposed to the rake face and a linear cutting edge portion (5b, 6b) extending so as to be in contact with the arcuate cutting edge portion (5a, 6a) are formed on an intersecting ridgeline portion between the rake face and the flank face so that the arcuate cutting edge portion (5a, 6a) and the linear cutting edge portion (5b, 6b) are alternately positioned in a circumferential direction of the rake face, a groove portion having a wall surface (8a, 8b) configured to be capable of coming into contact with a projection portion (14A, 14B, 14C) protruding from the bottom surface of the insert mounting seat is formed on the seating face, the groove portion has a narrow width portion in which a groove width decreases from one end side toward the other end side in a direction in which the groove portion extends, wherein one of the two cutting edges (5, 6) is a main cutting edge and the other is a minor cutting edge, the arcuate cutting edge portion of the minor cutting edge is formed to have a same radius as a radius of the arcuate cutting edge portion of the main cutting edge and have a shorter length than a length of the arcuate cutting edge portion of the main cutting edge in the circumferential direction, and the groove portion extends from an arcuate cutting edge portion side of the minor cutting edge toward a linear cutting edge portion side of the main cutting edge, characterized in that the linear cutting edge portion side of the main cutting edge is set as the one end side, and the arcuate cutting edge portion side of the minor cutting edge is set as the other end side.