1. A pattern recognizing device comprising: means for forming a structure pattern represented by date of a series of segments, based on a 2-dimensional pattern drawn with a single stroke, wherein said structure pattern forming means includes: (1) means for detecting segments where the relation between two segments adjacent to each other is ambiguous so that it is not possible to determine whether they form an arc or a segment of line, and (2) means for forming a plurality of patterns, each of which is comprised of data of a series of segments, both for the case where it is supposed that an ambiguous segment is an arc and for the case where it is supposed that an ambiguous segment is a segment of line, when at least one ambiguous segment is detected by said detecting means; means for storing a plurality of thesaurus structure patterns defined by data of a series of elements, each of which is at least one of an arc and a segment of line; and means for selecting one of said thesaurus structure patterns having the greatest similarity with a pattern obtained by said structure pattern forming a means, wherein said means for selecting includes: (1) first selecting means for selecting a corresponding most similar one of said thesaurus structure patterns for each of said plurality of patterns obtained by said means for forming a plurality of patterns, and (2) recognizing means for determining which one of said most similar thesaurus structure patterns and corresponding one of said plurality of patterns have the greatest similarity.