The purpose of this research is to examine the effect of a summer camp-based science methods course on elementary pre-service teachers' self-efficacy in teaching science as inquiry. The science camp was offered to K-6 students as part of the 4-week, 3-credit science teaching methods course over the summer. The pre-service teachers were asked to teach camp children for two weeks for their practicum requirement. This study utilized a mixed-methods design using both qualitative and quantitative data collected with 55 participants over four years. The TSI (Teaching Science as Inquiry) was administered at the beginning and end of the course to estimate participants' self-efficacy. We only measured PSTEB (Personal Science Teaching Efficacy Belief) using 34 items out of the TSI to reduce test fatigue. We also conducted semi-structured interviews at the end of the course to investigate sources for their self-efficacy. The paired samples t-test of the pre- and post-course survey indicates that pre-service teachers' self-efficacy in teaching science as inquiry increased significantly as a result of participating in the course (p < 0.001). Analysis of the interview data revealed eight main sources of self-efficacy and each one's relative significance compared to other sources. The findings of this study imply that the camp-based course increased pre-service teachers' self-efficacy by providing not only various mastery experiences, but also unique experiences that afforded reflection and mentoring, and drew camp participants' positive reactions. This study highlights the potential of informal science education settings as field experience sites especially for elementary science methods courses.