An insert adaptor for parting off including an adaptor index axis and parallel adaptor first and second sides connected by an adaptor peripheral surface which extends peripherally around the adaptor. The adaptor peripheral surface is formed with at least three pockets and bearing surfaces extending between the pockets. Each of the pockets comprises resilient upper and lower clamp surfaces.

A holder, a cutting tool and a machining method that allow coolant to be smoothly supplied to a machining location of a workpiece are provided. A holder for a cutting tool comprises: a plurality of mounting parts each of which is mounted with a cartridge that holds a cutting insert; and a coolant flow path that is opened in each mounting part.