1. A storing medium that stores a game BGM generating program to be executed by a game apparatus, said game apparatus comprising: a phrase data storing means for storing by each group based on a musical characteristic different kinds of a plurality of phrase data that designate a length and a pitch of a sound; a rhythm-pattern storing means for storing at least one kind of rhythm pattern data constructed of two or more rhythm data that designate a length in performance for performing a phrase, and a play timing of said phrase; a BGM-data playing means for playing BGM data constructed of at least one part; a sound-data storing means for storing data of the sound output according to said BGM data; and a sound outputting means for outputting a sound according to the BGM data reproduced by said BGM-data playing means, wherein said game BGM generating program allows a processor of said game apparatus to execute following steps of: a phrase selecting step for randomly selecting one kind of the phrase data from one group stored in said phrase data storing means; a rhythm selecting step for selecting one rhythm data from one kind of the rhythm pattern data stored in said rhythm-pattern storing means according to a predetermined rule; and a BGM generating step for generating the BGM data from the phrase data selected by said phrase selecting step and the rhythm data selected by said rhythm selecting step.