1. A method for analyzing the syntactic structure of a sentence used as an input to a natural language processing system, said method comprising the steps of: receiving the input sentence formed of a plurality of input words; retrieving and detecting the presence of defined words among a limited number of prestored words in a dictionary section of said natural language processing system in order to assign to the input words syntactic categories which are prestored together with the prestored words in said dictionary section, said defined words respectively corresponding to the input words; checking whether there exists among the input words any undefined words for which there are no corresponding defined words in said dictionary section; and assigning to an undefined word in the input sentence at least one syntactic category which satisfies grammar rules for the input sentence, independent of the dictionary consultation, said grammar rules being satisfied only when the syntactic categories of the input words are arranged in a predetermined pattern, thereby generating a syntactic analysis result of the input sentence; wherein said step of assigning includes: a step of assumptively selecting a reference syntactic category, from among a plurality of prepared reference syntactic categories, to assign said category to said undefined word included in the input sentence, said input sentence coinciding with a predetermined grammar rule; a grammar rule analysis step of determining whether a grammatical relationship between said undefined word applied to said one reference syntactic category in the input sentence and said input words satisfies said predetermined grammar rule; and a reference category reapplication step of assumptively selecting another reference syntactic category, from among the reference syntactic categories, to reassign the category to said undefined word when said another reference syntactic category applied to said undefined word has failed to coincide with the grammar rule of said input sentence in the configuration of the syntactic categories applied to said input words by said dictionary consultation.