The present invention provides a tool body (20) for use in an indexable rotary cutting tool, the tool body (20) having a rotational axis and a substantially cylindrical shape. Spiral grooves (28) are formed so as to extend from a base end side of the tool body to a leading end side in a peripheral surface (23) of the tool body. The spiral grooves (28) each have a plurality of insert seats (30) formed so as to be shifted from each other from the base end side toward the leading end side. The insert seats (30) each have a bottom surface (31) and first and second side surfaces (32, 33) that extend so as to intersect with the bottom surface (31). The first side surface (32) faces the base end side of the tool body (20), and the second side surface (33) faces the leading end side of the tool body (20) and is located on the base end side with respect to the first side surface (32). In at least one insert seat (30) located closer to the base end side with respect to an insert seat (30) located closest to the leading end side of the tool body (20), a length (L1) of the first side surface (32) is longer than a length (L2) of the second side surface (33).