To provide a long shaft inner surface machining device capable of making the maximum diameter of a machining head inserted into the long shaft thinner than lower holes of both end parts of the long shaft, preventing reduction of machining accuracy by reaction force of a tool at machining, minimizing a range of impossible machining, and accurately machining an inner surface of the long shaft along a lower hole.

The long shaft inner surface machining device is provided with a long shaft support device 10 for fixing the long shaft 1 so as not to be deflected; the machining head 20 capable of being inserted to the lower hole 2 of the long shaft 1 in an axial direction; a head support device 30 connected to the machining head 20 while passing through the lower hole 2 from one end of the long shaft 1 and moving the machining head 20 in an axial direction; and a cutter drive device 40 connected to the machining head while passing through the lower hole 2 from the other end of the long shaft 1 and rotation-driving the cutter 29 around an axis.