In an internal machining apparatus for machining an internal surface of a hollow workpiece, a machining unit holding a cutting blade is configured to be inserted into the hollow workpiece. A pair of unit holding arbors which are capable of non-rotatably holding the machining unit, inserted in the hollow workpiece, at both sides thereof are respectively supported by a work head and a foot stock to advance and retract in an axial direction thereof. A turning mechanism is provided to be supplied with a drive power transmitted through the interior of one of the unit holding arbors for turning the cutting blade held on the machining unit about an axis orthogonal to an axis of the unit holding arbors.