A rotary machining tool comprising a tool body incorporating a motor and a rotary drive shaft drivably connected to the motor; the rotary drive shaft comprising: a first shaft member having a base end drivably connected to the motor and further having a tip end; a second shaft member having a base end drivably connected to the tip end of the first shaft member and further having a tip end to which a machining member is attached; and an elastic member set between the first shaft member and the second shaft member to allow movement of the second shaft member toward the first shaft member while being elastically deformed; wherein the tip end of the first shaft member has a first sliding surface extending parallel to a longitudinal axis of the rotary drive shaft, and the base end of the second shaft member has a second sliding surface extending parallel to the longitudinal axis and slidingly engageable with the first sliding surface, so that sliding engagement of the first sliding surface and the second sliding surface allows the second shaft member to be displaceable relative to the first shaft member in a direction of the longitudinal axis and to be fixed relative to the first shaft member in a direction of rotation of the rotary drive shaft; wherein the tip end of the first shaft member has one of a projection and a recess which extend in the direction of the longitudinal axis and which are slidingly engageable with each other, the first sliding surface being formed on a side surface of the one of the projection and the recess, the base end of the second shaft member having an other of the projection and the recess, and the second sliding surface being formed on a side surface of the other of the projection and the recess; and characterised in that the projection is provided with a slot extending therethrough in a direction traversing the longitudinal axis and elongated in the direction of the longitudinal axis, the recess being provided with a shaft part extending in the direction traversing the longitudinal axis and received in the slot in the projection. a first shaft member having a base end drivably connected to the motor and further having a tip end; a second shaft member having a base end drivably connected to the tip end of the first shaft member and further having a tip end to which a machining member is attached; and an elastic member set between the first shaft member and the second shaft member to allow movement of the second shaft member toward the first shaft member while being elastically deformed; wherein the tip end of the first shaft member has a first sliding surface extending parallel to a longitudinal axis of the rotary drive shaft, and the base end of the second shaft member has a second sliding surface extending parallel to the longitudinal axis and slidingly engageable with the first sliding surface, so that sliding engagement of the first sliding surface and the second sliding surface allows the second shaft member to be displaceable relative to the first shaft member in a direction of the longitudinal axis and to be fixed relative to the first shaft member in a direction of rotation of the rotary drive shaft; wherein the tip end of the first shaft member has one of a projection and a recess which extend in the direction of the longitudinal axis and which are slidingly engageable with each other, the first sliding surface being formed on a side surface of the one of the projection and the recess, the base end of the second shaft member having an other of the projection and the recess, and the second sliding surface being formed on a side surface of the other of the projection and the recess; and characterised in that the projection is provided with a slot extending therethrough in a direction traversing the longitudinal axis and elongated in the direction of the longitudinal axis, the recess being provided with a shaft part extending in the direction traversing the longitudinal axis and received in the slot in the projection.