A machining unit that is supported on a machine tool and machines a workpiece by a revolving tool with a main spindle section being driven and rotated includes an eccentric rotational section, a tool holder, a tool revolution-radius changing mechanism, and a cutting-edge-orientation correcting mechanism. The eccentric rotational section is disposed on the main spindle section, and is rotatable about an eccentric axis that is located eccentrically at any distance in a radial direction of the main spindle section from a rotation center of the main spindle section. The tool holder is disposed on the eccentric rotational section, and supports the tool. The tool revolution-radius changing mechanism moves the tool in the radial direction, and changes the radius of the revolution of the tool by rotating the eccentric rotational section about the eccentric axis. The cutting-edge-orientation correcting mechanism corrects the orientation of a cutting edge of the tool by making the tool holder spin about an axis of the tool holder, the axis being parallel to the rotation center of the main spindle section.