This paper presents a self-generation piezoelectric sensor device for a wireless alarm system. A piezo-generator composed of a steel ball and piezoelectric vibrators senses the impact-induced vibration, and transforms its mechanical energy into electric energy to drive a wireless transmitter. First, the thickness and poling direction / electrode connection of the piezoelectric vibrator are examined to improve the efficiency of the piezo-generator. Next, the mechanical oscillations of a cantilever spring are used to repeat the impact and accumulate the electric energy up to the level enough for driving a micro-computer and a radio transmitter. A prototype wireless alarm system was assembled and worked successfully.