A cutting tool comprising a cutting insert and a tool body including an insert mounting part on which the cutting insert is removably mounted, wherein: the tool body includes a leading end surface and an outer side surface extending from the leading end surface to a base end side; the insert mounting part includes an opening provided in at least one of the leading end surface and the outer side surface ; and the insert mounting part includes a base surface and a side wall surface with which the cutting insert is brought into contact and second and third engagement parts which are formed in the base surface and with which the cutting insert is brought into contact, and wherein when, as viewed from a direction facing the base surface , the base surface is divided into four areas by a first boundary (L1) which is set so as to pass through substantially a center of the base surface and to extend from the leading end surface side to the base end side, and a second boundary (L2) which is set so as to pass through substantially the center of the base surface and to be orthogonal to the first boundary (L1), the second engagement part is formed in the first area on the outer side surface side relative to the first boundary (L1) and on the base end side relative to the second boundary (L2), the tool body includes a screw hole which is formed in the base surface so as to be screwed in by a fastening screw which fixes the cutting insert ; and when an intersection point between a central axis (H) of the screw hole and a virtual plane in which the base surface extends is regarded as a center point (X), characterised in that the third engagement part is formed in the second area on the outer side surface side relative to the first boundary (LI) and on the leading end surface side relative to the second boundary (L2). the tool body includes a leading end surface and an outer side surface extending from the leading end surface to a base end side; the insert mounting part includes an opening provided in at least one of the leading end surface and the outer side surface ; and the insert mounting part includes a base surface and a side wall surface with which the cutting insert is brought into contact and second and third engagement parts which are formed in the base surface and with which the cutting insert is brought into contact, and wherein when, as viewed from a direction facing the base surface , the base surface is divided into four areas by a first boundary (L1) which is set so as to pass through substantially a center of the base surface and to extend from the leading end surface side to the base end side, and a second boundary (L2) which is set so as to pass through substantially the center of the base surface and to be orthogonal to the first boundary (L1), the second engagement part is formed in the first area on the outer side surface side relative to the first boundary (L1) and on the base end side relative to the second boundary (L2), the tool body includes a screw hole which is formed in the base surface so as to be screwed in by a fastening screw which fixes the cutting insert ; and when an intersection point between a central axis (H) of the screw hole and a virtual plane in which the base surface extends is regarded as a center point (X), characterised in that the third engagement part is formed in the second area on the outer side surface side relative to the first boundary (LI) and on the leading end surface side relative to the second boundary (L2). <EOS>