1. A translation system for translating a document, comprising: a dictionary management unit for managing a plurality of categorized dictionaries classified according to predetermined categories; a phrase extraction unit for extracting a noun phrase from said document; a registration category selection unit for selecting a category into which said extracted noun phrase is registered among a plurality of categories corresponding to said plurality of categorized dictionaries, respectively; a translation unit for translating said noun phrase to generate a noun phrase translation which is a translation of said noun phrase; a dictionary registration unit for registering a pair of said noun phrase and said noun phrase translation on said categorized dictionary corresponding to the category selected by said registration category selection unit; a document category selection unit for selecting the category of said document on the basis of the frequencies of use of said plurality of categorized dictionaries in translation of said document; and a registration destination selection unit selects a category into which said extracted noun phrase is registered on the basis of the category selected by said document category selection unit.