South Korea is one of the countries with the longest working hours in the Organization for Economic Cooperation and Development (OECD) countries. According to the OECD annual report, the OECD Employment Outlook 2017, the average number of working hours in South Korea in 2016 was 2069 h, the second highest rate after Mexico [1]. The average annual working hours in the OECD 35 countries is 1764 h, and Korean workers are working 305 more hours than the average number of worked by workers in all OECD countries. There are studies on the effects of long hours of work on health, and most studies report that long hours of work negatively affect workers’ health [2]. Long working hours can be a risk factor of mental illness as well as cardiovascular disease [3, 4]. On the other hand, previous research suggests that job stress increases during long working hours [5], and job stress is associated with depressive symptoms [6, 7]. People with high job demands due to excessive workload or time pressure had twice the risk of major depression or generalized anxiety disorder compared to those with low job demands [6]. Also, Job stress due to high effort and low reward has had a negative impact on mental health [7]. However, there are few studies that explored the role of job stress factors between long working hours and depressive symptoms [8]. The first purpose of this study is to evaluate the association of long working hours on depressive symptoms. The second purpose is to determine the magnitude of job stress factors explaining the pathways of long working hours affecting depressive symptoms, taking into account job stress as mediator. Figure 1 shows an outline of the relationship between working hours and depressive symptoms. Socioeconomic position and irregular working time were seen as confounders. Job stress is considered as a mediator because, as work hours increase, the hours for workers to be exposed to job stress become longer and stronger, and the characteristics of the job needing long or short working hours could be related to job stress.