The first ferronickel manufacturing factory of the Republic of Korea was opened in Gwangyang City on October, 23rd, 2008. There has been public concern regarding heavy metal dust blown from the factory and slag disposal site. Therefore, we evaluated the health impact of the ferronickel factory on Gwangyang City residents by using interrupted time series analysis. We analyzed the monthly incidence patterns of asthma, allergic rhinitis, and dermatitis in Gwangyang City residents from 2004 to 2014. Data were gathered from the National Health Insurance Service database which covers all the hospital use data of entire city residents. Seasonality adjusted quasi-Poisson regression model was used to evaluate whether the operation of the ferronickel factory was associated with the immediate changes in the monthly disease incidence patterns. We set a control region, Yeosu City, near Gwangyang City to ensure that the changes in the disease incidence were specific to Gwangyang City. We conducted sub-regional level analysis to evaluate whether the disease incidence patterns were affected by the distance from the ferronickel factory. The risk estimates after operation of the ferronickel factory showed an abrupt increase in the monthly incidence of unspecified dermatitis [RR (95% CI), 1.75 (1.17-2.60)] and vasomotor and allergic rhinitis [RR (95% CI), 1.23 (1.08-1.39)] in men, and pruritus [RR (95% CI), 1.95 (1.51-2.52)], unspecified dermatitis [RR (95% CI), 1.65 (1.04-2.60)], and vasomotor and allergic rhinitis [RR (95% CI), 1.17 (1.04-1.31)] in women. These findings were significant even after accounting for the changes of the corresponding disease incidence of the comparison city, Yeosu. The effects were greater in young children (aged 0-9) and sub-regions near the ferronickel factory. Our study suggests possible association between the operation of the ferronickel factory and an abrupt increase of pruritus, unspecific dermatitis, and vasomotor and allergic rhinitis in Gwangyang City residents.