Abstract: A scanning system using tree structures is constituted by a propagation operating unit array having a tree structure in which propagation units are formed into groups on each layer, and each group of the propagation operating units is connected to a propagation operating unit belonging to the higher adjacent layer, and an interface element array connected to the propagation operating units of the lowermost layer. Each of the propagation operating units is constituted by a plurality of cascaded propagation elements. Each propagation element includes first and second selectors for selecting input signals DA.sub.i and DB.sub.i from the lower adjacent layer in accordance with two propagation signals from the immediately forwarding propagation element and a third selector for selecting two propagation signals from an adjacent propagation element in accordance with an input signal U from the upper adjacent layer.