Music is an important component of digital libraries. This paper discusses a digital music library from the information retrieval viewpoint and proposes a method for extracting theme phrases. These are then used to present a shorter version of retrieved music to users. The method consists of two steps, phrase extraction and syntactical classification of segmented fragments of melodies. Phrase extraction is carried out based on a few heuristic rules. We conducted an experiment on the accuracy of phrase extraction using 94 Japanese popular songs and obtained 0.766 recall and 0.786 precision. The syntactical classification is based on a probabilistic syntactical pattern analysis combining classification and syntactical analysis. The proposed method uses a decision tree and a finite state automaton and obtained 0.884 accuracy in theme phrase extraction.