The safety and reliability of automotive systems are becoming a big concern in our daily life. Recently, a functional safety standard which specializes in automotive systems has been proposed by the ISO. In addition, electrical throttle systems have been inspected by NHTSA and NASA due to the unintended acceleration problems of Toyota’s cars. In light of such recent circumstances, we are researching practical applications of formal methods to ensure the high quality of automotive operating systems. An operating system which we focus on is the one conforming to the OSEK/VDX standard. This paper shows a case study where model checking is applied to a commercial automotive operating system. In this case study, the model checking is combined with testing in order to efficiently and effectively verify it. As a result, we acquired the confidence that the quality of the operating system is very high.