To allow an engine assay and a suspension-module to be mounted together, by providing an attachable/detachable axle holder for holding a pair of axles at a prescribed position, when the suspension module having the pair of axles is mounted to the engine assay.

When a suspension module having a pair of axles A is mounted to an engine assay, attachable/detachable axle holders 14, 15 for holding the pair of axles A at a prescribed position are provided. When a engine assay having no axle A is mounted, the axle holders 14, 15 are removed not to interfere with the axles A mounted to a vehicle body side, and therefore, a pallet for loading the suspension module and a pallet for loading the engine assay can be shared. Only attachment or detachment of the axle holders 14, 15 is required and no replacement of the pallet is required, and a space for storing an excess pallet is not required.