1. An information retrieval system comprising: a speech input unit for inputting speech; an information storage unit for storing information with which speech information, of a length with which text degree of similarity is computable, is associated as a retrieval tag; an information selection unit for comparing a feature of each spoken content item extracted from each item of said speech information, with a feature of spoken content extracted from said input speech, to select information with which speech information similar to input speech is associated; an output unit for outputting information selected by said information selection unit, as information associated with input speech; a speech recognition unit for converting a speech signal into text data; a degree of similarity computation unit for computing degree of similarity between two or more speech recognition results generated by said speech recognition unit; and an information registration unit for registering a speech recognition result obtained by said speech recognition unit or a text feature vector generated from said speech recognition result, as said speech information associated with information that is to be retrieved, in said information storage unit; wherein speech recognition of said input speech is performed by said speech recognition unit; degree of similarity between a speech recognition result stored in advance as speech information in said information storage unit, and a speech recognition result of said input speech, is computed by said degree of similarity computation unit; and said information selection unit selects information with which speech information having a high degree of similarity to input speech is associated, based on degree of similarity computed by said degree of similarity computation unit.