There are provided a mobile device that is equipped with a musical composition-reproducing section and capable of curtailing the battery drain as much as possible, and a mobile device that prevents the quality of reproduced musical tones from being degraded even if the voltage level of the battery lowers. A battery voltage detector monitors an voltage level of a battery. The musical composition-reproducing section is controlled such that the number of parts of the musical composition reproduced by the musical composition-reproducing section is reduced when the monitored voltage level of the battery becomes lower than a reference value. In other forms of the invention, when the monitored voltage level of the battery becomes lower than a reference value, a first predetermined number of parts of the musical composition data are assigned to intrinsic ones of a larger second predetermined number of parts that correspond to the first predetermined number of parts of said musical composition, as well as to other ones of the second predetermined number of parts than the intrinsic ones, for reproduction of the musical composition, or the gain of a variable-gain amplifier is increased to compensate for the lowering of the volume of the reproduced parts.