Upon driving the unimorph type piezoelectric actuator of an ink jet in a high temperature high humidity environment (38°C//80%), many failures occurred after approximately 1 billion pulses. We hypothesized that the failures occurred due to hydrogen that was generated from the electrolysis of water, causing the piezoelectric element to deteriorate. Based on this hypothesis, we examined electrode materials to reduce the frequency of failures. We found that failure rate can be improved if material with high standard electrode potential is used as the electrode material on the high potential side and material with low standard electrode potential is used on the low potential side, and more specifically, if metal oxide is used for the electrode on the low potential side. Based on these findings, a combination of silver palladium for the electrode on the high potential side and stannous oxide Sn_xO_y for the low potential side was used as the combination of electrode materials that can be formed at low cost. Upon implementing a running test with this combination of electrode materials under a high temperature high humidity environment, we confirmed that failures do not occur even at a continuous drive of 30 billion pulses and that the long-term reliability of the unimorph type piezoelectric actuator improved significantly.