To improve the quality of a welded joint of a welding part to a horizontal beam of a receiving seat of a most important main electric motor.

A side plate 11a and a front surface plate 11b composing the receiving seat 11 of the main electric motor are made of separate members. When the receiving seat 11 of the main electric motor is welded on the horizontal beam 2, the front surface plate 11b can be lastly mounted on the horizontal beam 2 after members 11a and 11c to 11g except the front surface plate 11b are welded on the horizontal beam 2. As a result, problems wherein the welding part is difficult to be seen and the manipulation of a welding rod can not be performed as desired are eliminated when mounting the receiving seat 11 of the main electric motor on the horizontal beam 2, welding of the receiving seat 11 of the main electric motor on the horizontal beam 2 can be smoothly and surely performed, and the quality of welded joint is improved to increase reliability.