An indexable cutting insert has a first corner edge forming a front tip of a cutting edge portion configured to project furthest in the direction of an axis of rotation of an end mill tool receiving the insert. A major cutting edge is descending to come closer to the lower surface of the insert along its extension away from the first corner edge. The minor cutting edge extends from the first corner edge while descending from a point of the cutting edge portion most distant to the lower surface all the way to an inner corner of the insert and has a length of 80%-95% of the length of the major cutting edge including a second corner edge in a side elevation view of the cutting insert.