<p>In a signal amplifying device (10), a filter circuit (13) cuts off, among the high-frequency components included in an input signal, frequency components in a frequency band in which the output sound pressure level of a piezoelectric speaker (20) connected to the signal amplifying device (10) becomes below a predetermined value. An amplifier circuit (12) amplifies the output of the filter circuit (13) and supplies the amplified output to the piezoelectric speaker (20). A switching power supply circuit (11) boosts an externally supplied voltage up to a voltage necessary for driving the piezoelectric speaker (20) and supplies the boosted voltage to the amplifier circuit (12).</p>
