In order to perform turning operations of the kind that has heretofore been performed by diamond-shaped inserts, an insert which is basically equilateral triangular in shape with multiple alternately usable cutting edges, of which all three corners are usable as cutting edges, without decreasing the positional accuracy of the cutting edge during cutting. At least one recess and a plurality of protrusions are formed on each side of the insert. The recess and protrusions formed on the insert are brought into engagement with a protrusion and recesses formed on a side wall of a seating groove formed in a holder. In this state, the insert is pressed against a bottom seating face and the side wall of the seating groove with a clamp as a means to stably restrain the insert. This prevents shifting of the cutting edge during cutting even if the direction of the cutting force changes.