A tube cutting apparatus comprising: a disc-shaped inner blade rotatably supported on a rotating axis decentered by a predetermined length from an eccentric axis in a second direction which is opposite to a first direction of a tube in which the eccentric axis is decentered by the predetermined length from a main axis serving as a central axis of the tube within the tube, the predetermined length being larger than the difference in radius between the tube and the inner blade; first and second annular outer blades (4, 5) having respective inner peripheral surfaces for contact with an entire circumference of an outer periphery of the tube and placed in such a manner that their respective side surfaces which face each other in an axial direction of the tube are positioned substantially coincidently with opposite side surfaces of the inner blade in the axial direction; a driving source configured to output rotation; a main shaft having a free end side inserted into the tube and a base end side exposed exteriorly of the tube, the main shaft being supported on the base end side for rotation about the main axis and defining therethrough a bore decentered by the predetermined length from the main axis, the bore having open ends at opposite ends of the main shaft and a circular section; a columnar eccentric shaft rotatably supported in the bore and having a first end which is exposed from a free end of the main shaft within the tube and to which the inner blade is fixed in such a manner that a central position of the inner blade coincides with the main axis, and a second end exposed exteriorly of the tube; and rotation transmission means (9, 10) configured to transmit the rotation of the driving source to the base end side of the main shaft and to the second end of the eccentric shaft at a ratio of M to N, where M and N are different natural numbers, wherein the rotation transmission means includes: a first transmission section configured to transmit the rotation of the driving source to the main shaft at a first reduction gear ratio; characterized in that the rotation transmission means (9,10) further includes an intermediate shaft supported for rotation about the main axis; a second transmission section configured to transmit the rotation of the driving source to the intermediate shaft at a reduction gear ratio which is M/N as high as the first reduction gear ratio; and an offset shaft coupling for constantly transmitting rotation of the intermediate shaft to the eccentric shaft. a disc-shaped inner blade rotatably supported on a rotating axis decentered by a predetermined length from an eccentric axis in a second direction which is opposite to a first direction of a tube in which the eccentric axis is decentered by the predetermined length from a main axis serving as a central axis of the tube within the tube, the predetermined length being larger than the difference in radius between the tube and the inner blade; first and second annular outer blades (4, 5) having respective inner peripheral surfaces for contact with an entire circumference of an outer periphery of the tube and placed in such a manner that their respective side surfaces which face each other in an axial direction of the tube are positioned substantially coincidently with opposite side surfaces of the inner blade in the axial direction; a driving source configured to output rotation; a main shaft having a free end side inserted into the tube and a base end side exposed exteriorly of the tube, the main shaft being supported on the base end side for rotation about the main axis and defining therethrough a bore decentered by the predetermined length from the main axis, the bore having open ends at opposite ends of the main shaft and a circular section; a columnar eccentric shaft rotatably supported in the bore and having a first end which is exposed from a free end of the main shaft within the tube and to which the inner blade is fixed in such a manner that a central position of the inner blade coincides with the main axis, and a second end exposed exteriorly of the tube; and rotation transmission means (9, 10) configured to transmit the rotation of the driving source to the base end side of the main shaft and to the second end of the eccentric shaft at a ratio of M to N, where M and N are different natural numbers, wherein the rotation transmission means includes: a first transmission section configured to transmit the rotation of the driving source to the main shaft at a first reduction gear ratio; characterized in that the rotation transmission means (9,10) further includes an intermediate shaft supported for rotation about the main axis; a second transmission section configured to transmit the rotation of the driving source to the intermediate shaft at a reduction gear ratio which is M/N as high as the first reduction gear ratio; and an offset shaft coupling for constantly transmitting rotation of the intermediate shaft to the eccentric shaft. a first transmission section configured to transmit the rotation of the driving source to the main shaft at a first reduction gear ratio; characterized in that the rotation transmission means (9,10) further includes an intermediate shaft supported for rotation about the main axis; a second transmission section configured to transmit the rotation of the driving source to the intermediate shaft at a reduction gear ratio which is M/N as high as the first reduction gear ratio; and an offset shaft coupling for constantly transmitting rotation of the intermediate shaft to the eccentric shaft.