1. A computer-implemented conceptual network generating system that generates a conceptual network showing conceptual relations between words, the conceptual network generating system comprising: a first searching unit that searches a knowledge source storing search sentences, using as a search query first and second words conceptually related to each other, and retrieves a first search result sentence containing the first and second words; a first generating unit that analyzes the retrieved first search result sentence, and generates first structure information indicating words contained in the first search result sentence and a structure of the first search result sentence; a holding unit that stores the generated first structure information in a memory unit; a second searching unit that searches the knowledge source, using the first word as a search query, and retrieves a second search result sentence containing the first word; a second generating unit that analyzes the retrieved second search result sentence, and generates second structure information indicating words contained in the second search result sentence and a structure of the second search result sentence; a calculating unit that calculates similarity between the generated second structure information and the stored first structure information; and a setting unit that generates conceptual network information, based on the generated first structure information and second structure information having a similarity value equal to or larger than a first predetermined value with respect to the first structure information, the conceptual network information showing a conceptual relation between the first word and a third word contained in the second search result sentence corresponding to the second structure information, the conceptual relation between third word and the first word being equivalent to a conceptual relation between the second word and the first word.