Although methane plays an important role in climate change and atmospheric chemistry, its global budget remains quantitatively uncertain mainly because of a wide variety of source types. The stable carbon isotope ratio of atmospheric methane (delta C-13-CH4) is useful for separating contributions of different source categories, but owing to the complex and laborious analysis, limited measurement data exist. We present a new system for delta C-13-CH4 measurement, optimized for the automated analysis of air samples. Although the system is designed, in principle, similarly to those in previous studies, we successfully set up the system with no use of cryogen (e.g., liquid nitrogen) and attained reproducibility sufficient to analyze atmospheric variations (similar to 0.1 parts per thousand). We performed automated continuous measurements of ambient air outside our laboratory at about hourly intervals for 2 months, which characterized imprint of local methane sources well. Future measurement operation for flask air samples from existing atmospheric monitoring programs will provide a large number of atmospheric delta C-13-CH4 data.