A machine tool that generates internal teeth or external teeth on a cylindrical workpiece includes a workpiece holder, a column which is relatively movable to the workpiece holder, a rotary main spindle which is rotatable with respect to the column, a working tool which is held by the rotary main spindle, and a phase detecting unit which detects a phase of the working tool with respect to the rotary main spindle. The working tool includes a cutter portion which has cutters, a holder portion, and a detected portion which is formed on the holder portion. The phase detecting unit includes a detecting section which detects the detected portion, and a calculating section which calculates a phase angle of the detected portion with respect to a reference position of the rotary main spindle, based on a result detected by the detecting section.