A cutting insert (1, 21, 30) for an outer periphery of an end mill comprising: an insert main body having an upper surface and a plurality of side surfaces; a cutting edge located between both ends of a ridge which is between at least one side surface among the side surfaces and the upper surface; a high-positioned portion located at one end of the ridge; and a low-positioned portion located lower in a thickness direction of the insert main body than the high-positioned portion, and located at the other end of the ridge, wherein the cutting edge comprises: a first flat cutting edge having substantially the same height as the high-positioned portion; a second flat cutting edge having substantially the same height as the low-positioned portion; and a tilted cutting edge having a height decreasing from the high-positioned portion toward the low-positioned portion, and located between the first flat cutting edge and the second flat cutting edge, the side surface comprises: a first flank connecting to the first flat cutting edge; a second flank connecting to the tilted cutting edge; and a third flank connecting to the second flat cutting edge, the first flank and the third flank have clamping surfaces located on substantially the same flat surface, respectively, the cutting insert being characterised by a plurality of grooves (9a, 9b, 9c, 22a, 22b, 22c, 22d) extends from at least one side surface to the upper surface to divide the cutting edge by the grooves (9a, 9b, 9c, 22a, 22b, 22c, 22d), in the plurality of grooves (9a, 9b, 9c, 22a, 22b, 22c, 22d), the clearance angle within the groove (9c, 22d) nearest to the low-positioned portion is greater than the clearance angle within the groove (9a, 22a) nearest to the high-positioned portion, "a", "b" and "c" satisfy the relationship of a<b<c, wherein "a" represents the clearance angle within the groove formed at the first flank, "b" represents the clearance angle within the groove formed at the second flank, and "c" represents the clearance angle within the groove formed at the third flank. an insert main body having an upper surface and a plurality of side surfaces; a cutting edge located between both ends of a ridge which is between at least one side surface among the side surfaces and the upper surface; a high-positioned portion located at one end of the ridge; and a low-positioned portion located lower in a thickness direction of the insert main body than the high-positioned portion, and located at the other end of the ridge, wherein the cutting edge comprises: a first flat cutting edge having substantially the same height as the high-positioned portion; a second flat cutting edge having substantially the same height as the low-positioned portion; and a tilted cutting edge having a height decreasing from the high-positioned portion toward the low-positioned portion, and located between the first flat cutting edge and the second flat cutting edge, a first flat cutting edge having substantially the same height as the high-positioned portion; a second flat cutting edge having substantially the same height as the low-positioned portion; and a tilted cutting edge having a height decreasing from the high-positioned portion toward the low-positioned portion, and located between the first flat cutting edge and the second flat cutting edge, the side surface comprises: a first flank connecting to the first flat cutting edge; a second flank connecting to the tilted cutting edge; and a third flank connecting to the second flat cutting edge, the first flank and the third flank have clamping surfaces located on substantially the same flat surface, respectively, the cutting insert being characterised by a plurality of grooves (9a, 9b, 9c, 22a, 22b, 22c, 22d) extends from at least one side surface to the upper surface to divide the cutting edge by the grooves (9a, 9b, 9c, 22a, 22b, 22c, 22d), in the plurality of grooves (9a, 9b, 9c, 22a, 22b, 22c, 22d), the clearance angle within the groove (9c, 22d) nearest to the low-positioned portion is greater than the clearance angle within the groove (9a, 22a) nearest to the high-positioned portion, "a", "b" and "c" satisfy the relationship of a<b<c, wherein "a" represents the clearance angle within the groove formed at the first flank, "b" represents the clearance angle within the groove formed at the second flank, and "c" represents the clearance angle within the groove formed at the third flank.