Aim Decreased frequency of going outside the home and being socially isolated are regarded as predictors of poor health. The object of the present study was to clarify whether these factors have synergistic or independent impacts on future functional decline. Methods We examined a prospective cohort of 2427 community‐dwelling persons, aged ≥65 years, who responded to the baseline mail survey in Wako City, in 2008. Participants were asked about the frequency of going outside the home, social isolation status (having contact less than once a week with anyone outside household), functional capacity (Tokyo Metropolitan Institute of Gerontology‐Index of Competence), age, sex, annual income, self‐rated health, depressive mood and mobility. Results Of 1575 persons (72.1%) who completely responded to the follow‐up survey (T2) in 2012, we defined the groups as follows: group 1, not isolated and going outside the home every day (n = 897); group 2, not isolated and going outside the home less than every day (n = 311); group 3, isolated and going outside the home every day (n = 224); and group 4, isolated and going outside the home less than every day (n = 143). Multiple logistic regression analyses showed that the variables identifying group 3 for men and group 2 for women with reference to group 1 were predictors of subsequent functional decline even after adjustment for confounders (odds ratios 2.01, 1.63; 95% CI 1.20–3.38, 1.03–2.56, respectively). Conclusion Social isolation regardless of going outside the home every day for men and going outside the home less than every day regardless of being not socially isolated for women might predict functional decline. Geriatr Gerontol Int 2017; 17: 500–508.