A rotary cutting tool which cuts a workpiece through rotation, the rotary cutting tool comprising: a plurality of cutting inserts each having a cutting edge; a plurality of cartridges each having an insert mounting seat on which the cutting insert is mounted; and a body having a first cartridge mounting seat and a second cartridge mounting seat at positions separate from each other in a circumferential direction of a predetermined axis, the first cartridge mounting seat and the second cartridge mounting seat each having the cartridge mounted thereon, wherein: the cartridge has a protrusion in an outer side surface of the cartridge; the first cartridge mounting seat has: a first wall surface opposing the outer side surface of the cartridge mounted on the first cartridge mounting seat; the second cartridge mounting seat has: a second wall surface which faces in a same direction as the first wall surface and which opposes the outer side surface of the cartridge mounted on the second cartridge mounting seat; characterised by a first recessed part which is formed in the first wall surface, into which the protrusion is inserted, and which comes into contact, at a bottom thereof, with a leading end of the protrusion, and further characterised by a second recessed part which is formed in the second wall surface, into which the protrusion is inserted, and which comes into contact, at a bottom thereof, with the leading end of the protrusion; a depth of the second recessed part is greater than a depth of the first recessed part; and an amount of protrusion of the protrusion is equal to or greater than the depth of the second recessed part. a plurality of cutting inserts each having a cutting edge; a plurality of cartridges each having an insert mounting seat on which the cutting insert is mounted; and a body having a first cartridge mounting seat and a second cartridge mounting seat at positions separate from each other in a circumferential direction of a predetermined axis, the first cartridge mounting seat and the second cartridge mounting seat each having the cartridge mounted thereon, wherein: the cartridge has a protrusion in an outer side surface of the cartridge; the first cartridge mounting seat has: a first wall surface opposing the outer side surface of the cartridge mounted on the first cartridge mounting seat; the second cartridge mounting seat has: a second wall surface which faces in a same direction as the first wall surface and which opposes the outer side surface of the cartridge mounted on the second cartridge mounting seat; characterised by a first recessed part which is formed in the first wall surface, into which the protrusion is inserted, and which comes into contact, at a bottom thereof, with a leading end of the protrusion, and further characterised by a second recessed part which is formed in the second wall surface, into which the protrusion is inserted, and which comes into contact, at a bottom thereof, with the leading end of the protrusion; a depth of the second recessed part is greater than a depth of the first recessed part; and an amount of protrusion of the protrusion is equal to or greater than the depth of the second recessed part. a second wall surface which faces in a same direction as the first wall surface and which opposes the outer side surface of the cartridge mounted on the second cartridge mounting seat; characterised by a first recessed part which is formed in the first wall surface, into which the protrusion is inserted, and which comes into contact, at a bottom thereof, with a leading end of the protrusion, and further characterised by a second recessed part which is formed in the second wall surface, into which the protrusion is inserted, and which comes into contact, at a bottom thereof, with the leading end of the protrusion; a depth of the second recessed part is greater than a depth of the first recessed part; and an amount of protrusion of the protrusion is equal to or greater than the depth of the second recessed part.