Introduction The proportion of atypical employment has increased in Japan since the economic crisis of the 1990s. Previous studies have reported that non-permanent employees tend to be psychologically distressed. However, the relationship between occupational stress and non-permanent workers’ mental health is unclear. We investigated their job stress and psychological distress.

Methods We surveyed 86 non-permanent employees at a municipal office using the Effort–Reward Imbalance Questionnaire, Kessler 6 (K6), and Job Content Questionnaire (JCQ). Employees were divided into two groups based on an effort–reward ratio (ER ratio) cutoff of 1, median reward scale scores (financial, esteem-related, and organisational reward), and median JCQ subscale scores (job demand, job control, and social support). We employed multi-way analysis of variance. In the first analysis, the dependent variable was the K6 score, and the independent variables were ER ratio, JCQ score, age, and gender. In the second analysis, we added three reward scales to the independent variables of the first analysis. The statistical significance level was set at 5%.

Result In the first analysis, the main effect of ER ratio was marginally significant [F(1, 75)=3.08, p=0.0832]. The least square mean of K6 scores was 5.54 in the high-ER ratio group and 3.28 in the low-ER ratio group. In the second analysis, no main effect was observed.

Discussion We hypothesised that non-permanent employees with low ER ratios would have better mental health than those with higher ER ratios. The result of the first analysis did not support this hypothesis, although it suggests this tendency. The second analysis showed no relationship between external reward and psychological distress. Thus, avoiding ER imbalance can lead to good mental health. The study limitations include the cross-sectional design and the lack of information about marital status and education level. Therefore, further investigation should be conducted.