Abstract: A method for erection by launching of prefabricated modular deck of viaducts, motorway flyovers and the like and a mechanism for performing the method. The mechanism is a mobile bridge including towers and a replaceable platform supported by the towers. Each tower consists of a set of telescopic sections moveable up and down, placed on a track. Carrying capacity and structure of the bridge provide supporting and lowering the deck module and its carrier. The platform which length is less than a distance between the viaduct piers has expandable ends. To increase stability of the bridge during the erection process the platform can be temporarily affixed to the viaduct. The method is performed as a set of steps. Steps 1 and 2--installing the bridge between viaduct piers to provide a continuous way from an existing part of the viaduct to the bridge. Steps 3 and 4--delivering the module to the place of erection using an existing part of the viaduct as a road.