The invention provides a gear machining device and a gear machining method. The gear machining device is provided with a support part which enables the machined gear to be installed in a freely installed and detached manner, a first workpiece support unit which support the support part in a freely installed and detached manner and enables the support part to rotate, a workpiece moving unit which enables the machined gear to be away from the support part, a detection part which a rotating position of the machined gear supported by the support part, a control part which controls the first workpiece support unit, a workpiece conveying unit which enables the support part to move from the first workpiece support unit to a machining position, a second workpiece support unit which support the support part in freely installed and detached manner and enables the support part to rotate, and a cutter unit provided with a cutter one edge of which is engaged with the machined gear supported on the second workpiece support unit and one edge, performing rotating, of which machines the machined gear. The support part and the cutter are controlled by utilizing the control part, and then the support part and the cutter perform synchronous rotating based on a specified phase difference.