Abstract: This invention relates to improvements of a piezoelectric acoustic device used for a warning buzzer, general buzzer, etc. The conventional piezoelectric acoustic device generates relatively small sound volume, and has disadvantages not sufficiently used practically. This invention contemplates to eliminate this disadvantage, and has a feature that a composite plate for generating a sound is supported in a casing having an opening provided in the direction of thickness, i.e. direction of sound propagation, of the composite plate, and a reflecting plate for reflecting the sound is provided at the rear surface side. Another feature of the present invention is that the above reflecting plate is formed in horn shape to further amplify the generated sound. This invention thus provides an acoustic device for generating large volume with compact structure and less power of consumption.