1. A work transfer system including a plurality of work transfer units which connect a plurality of loading places to a plurality of unloading places, the system comprising: a layout information database which stores layout information representing a layout of the plurality of work transfer units; and route setting means implemented within a computer for setting, based on the layout information, a route of the work transfer units to pass a work between a planned loading place and a planned unloading place which are required to transfer the work in the plurality of loading places and the plurality of unloading places, wherein the plurality of work transfer units are constituted from a plurality of first work transfer units which transfer the work in one direction and a plurality of second work transfer units capable of changing a work transfer direction, the first and second work transfer units transferring the work by receiving and sending the work between two adjacent work transfer units, said route setting means comprising: candidate setting means for setting a plurality of candidates of the route between the planned loading place and the planned unloading place; and route selection means for selecting, based on a predetermined condition, one route from the plurality of candidates of the route set by said candidate setting means, said route selection means calculating a route selection evaluated value by 
 Route selection evaluated value={2/(1+ e −a(Tw/Tnw−1) )}−1 where Tnw is a transfer time without interference with another work, Tw is a transfer time including the wait time for avoiding interference with another work which has an already set route, and a is a slope of a sigmoid function; and said candidate setting means sets, as a via point, each of the plurality of work transfer units capable of passing the work between the planned loading place and the planned unloading place and sets a shortest route as the candidate of the route for each via point.