Disclosed herein is a jig for assembling a rotor. The jig includes an upper jig part, a lower jig part, a support plate, a guide jig part, and a spring. The upper jig part has a pressure protrusion for press-fitting a rotating shaft, and applies pressing force to the rotating shaft. The lower jig part supports a rotor casing in a direction which is opposite the acting direction of the pressing force. The support plate is mounted to the lower jig part, and supports the rotor casing in a direction which is opposite that of the force for press-fitting the rotating shaft. The guide jig part supports the rotor casing in a direction which is opposite that of reaction force generated in the rotor casing by the support plate. The spring elastically biases the guide jig part towards the rotor casing.