After workers take long-term sickness absence due to mental disorders (LTSA-MD), the occupational stress of the coworkers in the same work unit might be affected. The aims of this study were to evaluate the effect of the incident of LTSA-MD on the coworkers' occupational stress. A retrospective cohort study of 16,032 public servants was conducted. The Brief Job Stress Questionnaire (BJSQ) was used, which was administered in 2011 and 2012. To analyze the amount of change in occupational stress, the difference between the scores of the BJSQ scales in 2011 and 2012 was calculated. After adjusting for the baseline BJSQ scales, sex, age, total number of workers, and social support, analysis of covariance of the difference between the BJSQ scales' scores showed that job stressors and stress responses worsened among the coworkers after the incident of LTSA-MD. Social support did not change among the coworkers. This study indicates that an incident of LTSA-MD in the same work unit adversely affects the coworkers' occupational stress. Focusing on the coworkers' mental state after an incident of LTSA-MD in the same work unit and an early intervention strategy are needed to prevent secondary mental illness and sickness absence in the coworkers.