In recent years, many various energy sources have been investigated as replacements for traditional automotive fossil fuels to help reduce CO2 emissions, respond to instabilities in the supply of fossil fuels, and reduce emissions of air pollutants in urban areas. Toyota Motor Corporation considers the plug-in hybrid vehicle (PHV), which can efficiently use electricity supplied from infrastructure, to be the most practical current solution to these issues. For this reason, Toyota began sales of the Prius Plug-in Hybrid in 2012 in the U.S., Europe and Japan. This is the first PHV to be mass-produced by Toyota Motor Corporation. Prior to this, in December 2009, Toyota sold 650 PHVs through lease programs for validation testing in the U.S., Europe and Japan. Additional 30 PHVs were introduced in China in March 2011 for the same objective. As a result, it is confirmed not only fuel reduction effect in case of real market usage, but also relationship between frequency of charging and fuel reduction effect. The relationship between frequency of charging and fuel reduction effect is the specific performance not shared by both conventional gasoline vehicle and electric vehicle. It is also confirmed that improvement of charging convenience contributes to fuel reduction effect. The system of the mass-produced vehicle underwent major improvements in response to the results of this validation testing. Due to this, Toyota succeeded in developing the system which suits the actual environment of usage. This paper discusses the analysis of this validation test results.