The stable carbon isotope ratio (δ13C) of low-volatile water-soluble organic carbon (LV-WSOC) was measured in filter samples of total suspended particulate matter, collected every 24 h in the winter of 2010 at an urban site and two rural sites in western Japan. Concentrations of the major chemical species in fine aerosol (<1.0 μm) were also measured in real time by aerosol mass spectrometers. The oxidation state of organic aerosol was evaluated using f44; i.e., the proportion of the signal at m/z 44 (CO2+ ions from the carboxyl group) to the sum of all m/z signals in the organic mass spectra. A strong correlation between LV-WSOC and m/z 44 concentrations was observed, which suggested that LV-WSOC was likely to be associated with carboxylic acids in fine aerosol. Plots of δ13C of LV-WSOC versus f44 showed random variation at the urban site and systematic trends at the rural sites. The systematic trends qualitatively agreed with a simple binary mixture model of secondary organic aerosol with background LV-WSOC with an f44 of ∼0.08 and δ13C of -17‰ or higher. Comparison with reference values suggested that the source of background LV-WSOC was likely to be primary emissions associated with C4 plants.