While the amplification factor of an external amplifier is increased to raise the minimum volume level, synthesized speech of text data is generated as PCM data. When the PCM data is output for each frame (63 ms), the voltage of a battery power supply is detected and different ATT values (attenuation ratios) are set in accordance with threshold values (Level 1.about.4) corresponding to the detected voltage. The lower the power supply voltage, the larger the attenuation ratios. The PCM data is attenuated by such an attenuation ratio to reduce a load to the battery power supply. Even though the battery power supply is exhausted to some extent, no power-down due to an instantaneous increase in power load occurs, with the result that a high-level part of a series of speech data items is output at an adequate volume level that makes a low-level part thereof easy to hear.