A cutting tool assembly including: (a) an arbor including a tapered shank portion which has a taper of 7/24 and a size of No. 30; (b) a cutting tool having at least one cutting blade and fastened to the arbor; (c) a fastener for fastening the cutting tool to the arbor; and (d) a relative-rotation preventing mechanism for preventing rotation of the cutting tool relative to the arbor. The cutting tool assembly has a weight of not larger than 3 kg. The arbor and the cutting tool have respective contact surfaces which are held in contact with each other. The relative-rotation preventing mechanism is provided by at least one receiving hole which is formed in one of the contact surfaces, and at least one protrusion which is formed on the other of the contact surfaces and which is received in the above-described at least one receiving hole.