An insert rotary cutting tool includes a tool body of a generally circular cross-section having an axis of rotation therethrough and having an outer periphery. The cutting tool has plural groups of cutting inserts releasably mounted on the outer periphery of the body with the groups being spaced circumferentially of the body from each other. The inserts of each group are displaced circumferentially of the body and spaced axially of the body. Each of the inserts has a plurality of main cutting edges and is disposed in such a manner that one of the main cutting edges is indexed in a peripheral cutting position to serve as a peripheral cutting edge. The indexed peripheral cutting edges of the inserts are in series axially of the body as viewed circumferentially of the body. The indexed peripheral cutting edges of the inserts of each group have foremost ends disposed on a common helical line extending helically axially of the body. The helical line which the foremost ends of the indexed peripheral cutting edges of the inserts of at least one of the groups are disposed on is provided with a helix angle different from helix angles of the helical lines which the foremost ends of the indexed peripheral cutting edges of the inserts of the other groups are disposed on.