1. A keyword input supporting apparatus comprising: a document acquisition unit which acquires a document having a plurality of components containing text data; a main component selection unit which selects a component having many characters in the text data among the components as a main component; an analysis unit which analyzes the part-of-speech of the text data contained in the main component, and adds a semantic attribute to each of words of the analyzed text data; an extraction unit which extracts as a specific name a word, having one of a predetermined semantic attribute and a predetermined part of speech, from each of the words; a storage unit which stores the specific name together with a corresponding semantic attribute; a keyword candidate classification unit which performs classification of the specific name from the storage unit as a keyword candidate based on the semantic attribute; and a presentation unit which presents the keyword candidate to a user.