1. A cross-layer routing method in a wireless sensor network comprising: locating a destination node which is a final destination of data by a source node which generates the data; and determining an optimal relay node among one or more neighboring nodes using their position information for each hop and forwarding the data from the source node to the destination node through the determined relay nodes, wherein determining an optimal relay node and forwarding the data comprises: broadcasting data by a sending node; forwarding the data through broadcasting first by a node determined as a winner node from among neighboring nodes that receive the broadcasting of the sending node; canceling data forwarding by nodes in a transmission range of the winner node after receiving the broadcasting of the winner node; and canceling data forwarding of the neighboring nodes that do not cancel data forwarding, by the sending node after receiving the broadcasting of the winner node, wherein forwarding the data through broadcasting first by a node determined as a winner node comprises: waiting for data relaying by calculating contention windows according to priorities by the neighboring nodes; and forwarding data through broadcasting first by a winner node that calculates the lowest contention window among the neighboring nodes, and wherein the contention window value includes a progress obtained by projecting a distance between the sending node and a receiving node onto a straight line connecting the sending node to the destination node, the least distance between the receiving node and the straight line connecting the sending node to the destination node, a transmission range of the sending node, and the maximum contention window value of the receiving node.