Abstract: Front retaining frame (15) retains a front drive-train component part assembly (35) having front suspensions (38) combined with an engine (36) in advance, and a rear retaining frame (18) retains a rear drive-train component part assembly (47) having rear suspensions (51) combined with a rear wheel axle (49) in advance. Elevated transport path (11, 14, 17) transports the front and rear retaining frames (15, 18), having the front and rear drive-train component part assemblies retained thereby in a hung-down state, with the front and rear retaining frames detachably attached to a retaining frame positioning member. By use of the front and rear retaining frames, human operators can perform all or most of necessary operations in a downwardly-looking posture, which can improve a working environment.