A cutting edge-replaceable cutting tool (10, 100) comprising a tool body, on the tool body cutting inserts being removably installed, the cutting tool comprising: a plurality of insert mounting seats formed on the tool body; and a plurality of upper wall portions formed integrally with the tool body, each of the plurality of upper wall portions overhanging a corresponding insert mounting seat of the plurality of insert mounting seats; wherein the cutting insert comprises two opposite end surfaces (50, 52), a circumferential side surface extending between the end surfaces, and two cutting edges each formed at an intersection ridge portion between one of the end surfaces (50, 52) and the circumferential side surface; the insert mounting seat comprising: a bottom wall surface on which one of the two opposite end surfaces (50, 52) of the cutting insert is mounted; and a side wall surface with which the circumferential side surface of the cutting insert comes into contact; wherein the side wall surface is formed integrally with a portion which forms the bottom wall surface of another insert mounting seat adjacent to the insert mounting seat; wherein the cutting insert is mounted on the insert mounting seat such that the cutting insert is surrounded by the bottom wall surface, the side wall surface, and the upper wall surface, and each of the upper wall portion is formed so as to overhang from the side wall surface toward the outer circumference of the tool; the upper wall portion includes an upper wall surface located opposite the bottom wall surface and the upper wall portion overhangs so as to cover one unused cutting edge of the cutting insert when the cutting insert is mounted on the corresponding insert mounting seat, and the upper wall portion is formed so as to increase an amount by which the upper wall portion overhangs from the side wall surface ( of a portion for supporting the cutting insert from the leading end side to base end side with respect to the insert mounting seat adjacent to the corresponding insert mounting seat, and the amount of overhang of the upper wall portion increases progressively from the leading end side to the base end side of the insert mounting seat, characterised in that the distance from the end of the upper wall portion to the bottom wall surface of another insert mounting seat adjacent to the insert mounting seat increases as the amount of overhang of the upper wall portion increases progressively according to the increase of the amount. a plurality of insert mounting seats formed on the tool body; and a plurality of upper wall portions formed integrally with the tool body, each of the plurality of upper wall portions overhanging a corresponding insert mounting seat of the plurality of insert mounting seats; the cutting insert comprises two opposite end surfaces (50, 52), a circumferential side surface extending between the end surfaces, and two cutting edges each formed at an intersection ridge portion between one of the end surfaces (50, 52) and the circumferential side surface; the insert mounting seat comprising: a bottom wall surface on which one of the two opposite end surfaces (50, 52) of the cutting insert is mounted; and a side wall surface with which the circumferential side surface of the cutting insert comes into contact; wherein the side wall surface is formed integrally with a portion which forms the bottom wall surface of another insert mounting seat adjacent to the insert mounting seat; a bottom wall surface on which one of the two opposite end surfaces (50, 52) of the cutting insert is mounted; and a side wall surface with which the circumferential side surface of the cutting insert comes into contact; wherein the side wall surface is formed integrally with a portion which forms the bottom wall surface of another insert mounting seat adjacent to the insert mounting seat; wherein the cutting insert is mounted on the insert mounting seat such that the cutting insert is surrounded by the bottom wall surface, the side wall surface, and the upper wall surface, and each of the upper wall portion is formed so as to overhang from the side wall surface toward the outer circumference of the tool; the upper wall portion includes an upper wall surface located opposite the bottom wall surface and the upper wall portion overhangs so as to cover one unused cutting edge of the cutting insert when the cutting insert is mounted on the corresponding insert mounting seat, and the upper wall portion is formed so as to increase an amount by which the upper wall portion overhangs from the side wall surface ( of a portion for supporting the cutting insert from the leading end side to base end side with respect to the insert mounting seat adjacent to the corresponding insert mounting seat, and the amount of overhang of the upper wall portion increases progressively from the leading end side to the base end side of the insert mounting seat, characterised in that the distance from the end of the upper wall portion to the bottom wall surface of another insert mounting seat adjacent to the insert mounting seat increases as the amount of overhang of the upper wall portion increases progressively according to the increase of the amount.