The present disclosure improves the machining conditions and enhances the quality of machined surfaces, particularly in cutting off and grooving machining when the projection amount of a plate-like member is large. A blade, which is a plate-like member having a polygonal shape and used for cutting machining, includes: a plurality of insert pockets; and recess portions formed by partial cutting corresponding to insert pockets respectively. A holder includes: a holder main portion; and a support portion with which a part of the recess portion formed in the blade contacts. The holder is configured in which the blade can be installed in a state where the part of the recess portion can contact with the support portion.