To provide a tool capable of, when grinding inner surfaces of a plurality of machining target portions, increasing its machining efficiency by simultaneously machining the machining target portions while ensuring its machining accuracy at all the machining target portions.

An inner surface grinding tool 1 is provided with a plurality of coaxially disposed machining units 3, wherein: a core shaft 7 capable of freely advancing and retreating by being driven by a core shaft motor 16 is disposed at a central part of a tool axis of each machining unit 3, a plurality of male-threaded members 8 is disposed at a prescribed position of the core shaft 7, a cylindrical slide slop member 9 having a female-threaded portion 9m is disposed on the outside of the male-threaded members 8, and the threaded portions are meshed with each other. Each cutting tool 5 expands and contracts as the slide slope member 9 advances and retreats, and a pinion gear 21 of an individual drive mechanism 18 can be meshed with a face gear 8f of each male-threaded member 8.