Due to the recent popularization of the Geographic Information System (GIS), spatial network environments that can display the changes of spatial axes on mobile phones are receiving great attention. In spatial network environments, since a query object that seeks location information selects several candidate target objects based on the search conditions, we often use a k-nearest neighbor (kNN) search, which seeks several target objects near the query object. However, since a kNN search needs to find the kNN by calculating the distance from the query to all the objects, the computational complexity might become too large, depending on the number of objects. To reduce this computation time in a kNN search, many researchers have proposed a search method that divides regions using a Voronoi diagram. In this method, we reduce the computational complexity by generating Voronoi regions for each object based on the positional relationships between two objects. In addition, a search method was proposed to reduce the processing time by generating Voronoi diagrams using a contact zone. However, since conventional methods generate Voronoi diagrams for objects in order, the computational complexity might become too large, depending on the number of objects. In this paper, we propose a generation method of the Voronoi diagram to the reduce processing time by parallelizing the generation of Voronoi regions using a contact zone. In our evaluation, we confirmed that the processing time under the proposed method was reduced about 15.9% more than conventional methods that are not parallelized.