Self-efficacy is an important construct for CS teachers' professional development, because it can predict both teaching behaviors as well as student outcomes. Research has shown that teachers' self-efficacy can be as influential as their actual level of knowledge and abilities. However, there has been very limited research on CS teachers' self-efficacy. This study describes the development and implementation of an instrument that measures secondary school teachers' self-efficacy in teaching computer science. Teachers attended a nine-week hybrid professional development program and completed the computer science teaching self-efficacy instrument. Confirmatory factor analysis validated the self-efficacy instrument, which can be potentially used in other CS education settings. The results also indicated that teachers' self-efficacy in the content knowledge and pedagogical content knowledge dimensions of teaching computer science significantly increased from participating in the professional development program.