To investigate the influences of a merger on employees in a Japanese company, changes of subjective stress and stress-related symptoms after a merger announcement in a major Japanese financial company were explored using longitudinal study surveys. Seventy-one participants responded to the first and second questionnaires, consisting of stress and symptoms, personal characteristics, lifestyle, medical examination, and work-related factors. After the merger announcement, the prevalence of subjective stress, anxiety, and impatience increased significantly from 46.5% to 78.9%, 18.3% to 40.8%, and 15.9% to 29.0%, respectively. The study suggests that subjective stress and stress-related symptoms may increase after a merger announcement. To reduce the negative impact of mergers, employers are expected to provide mental health-promotion programs in a merger-planning company.