The purpose of this study was to examine the effects of social identity on psychological distress in areas where residents had been evacuated to after the Fukushima nuclear accident. In a door-to-door survey, we surveyed 74 evacuees and 81 host community residents in a relocation area in Fukushima to evaluate whether social support mitigated evacuees’ and hosts’ distress and whether the social identity of each community affected distress. Social support mitigated distress in both evacuees and hosts. The construction and effect of social identity differed between groups: Evacuees had high original community (ingroup) identity, and their host community (outgroup) identity decreased their distress. Hosts had high host community (ingroup) identity but low evacuee community (outgroup) identity. Hosts with high ingroup identity who integrated outgroup identity had increased distress. These results suggest that management of social identity in addition to social support is important for mitigating psychological distress after the nuclear accident, and that support for individuals should be focused on management of both host and evacuee communities in relocation areas.