A gear machining device is provided which can machine an accurate gear by a cutting process by synchronously rotating a machining tool and a workpiece at a high speed. A machining tool includes roughing cutting teeth for roughing bottom lands and right and left side faces of teeth of a gear, right side face finishing cutting teeth for finishing the right side faces of the teeth, and left side face finishing cutting teeth for finishing the left side faces of the teeth. The cutting teeth thus cut different parts of the teeth of the gear. This can reduce cutting resistance and suppress self-vibration during the cutting process, thereby improving tooth trace accuracy of the gear.