1. A structured-text cataloging/searching method for a text searching system, in which a set of texts is searched for specific text contents, comprising the following steps: an already-analyzed-text data generating/cataloging step of cataloging, in a text database, already-analyzed-text data obtained from an analysis of a logical structure of a text to be cataloged, said already-analyzed-text data generating/cataloging step being performed for a plurality of texts to be cataloged; a structure-index creating step of creating a structure index, by sequentially superposing logical structures of said plurality of texts cataloged in said already-analyzed-text data generating/cataloging step; wherein said structure index has a tree-like structure composed of a plurality of metanodes; wherein a context identifier that uniquely identifies one of said metanodes is assigned to each metanode of said structure index; and wherein a group of structure elements having the same position of appearance and the same element type for a plurality of texts are represented by a single metanode; a character-string-index updating step comprising the sub-steps of: extracting partial character strings each having a predetermined character count from each of a plurality of texts to be cataloged; and updating a character string index by cataloging an associative relation between each of said partial character strings and structured character position information of that partial character string in said character string index; a structure-condition judging step of searching the structure index for a set of context identifiers satisfying a specific structure condition; a structured-character-position-information extracting step of extracting partial character strings from a search term, each extracted partial character string having a predetermined character count, and searching the character string index for a set of pieces of structured-character-position information matching said extracted partial character strings; and an index searching step of searching said set of pieces of structured-character-position information for specific pieces of structured-character-position information that have context identifiers found at said structure-condition judging step, and that have a positional relation among said specific pieces of structured-character-position information matching an order of arrangements of said partial character strings in said search term.