The Zung self-rating depression scale (SDS) was applied to Japanese night shift workers of a railway company in order to assess the prevalence of depressive state and clarify the structural characteristics of the SDS questionnaire. Out of 2,394 employees, 1,931 male workers were engaged in night shift work and 1,274 men (66.0%) completely answered the questionnaire. The control group was selected from daytime workers. The average age of the subjects was 39.4. Cronbach's alpha coefficient of the night shift workers was 0.835 and that of the control was 0.848. SDS index decreased with age. The mild, moderate, and severe depressive state of night shift workers evaluated by SDS were 13.7%, 2.1%, and 0.6%, respectively. A two-way analysis of variance on SDS index examined age and shift work. Age influenced SDS index, but there was no interaction between the factors. Two factors were extracted by factor analysis. Factor 1 consisted of 8 positive items, accounting for 16.7%. Factor 2 consisted of 8 depressive items, accounting for 15.6%. The residual four items consisted of somatic or behavioral features. Mean value of SDS index by age, distribution of SDS index, and factor structure of SDS questions of the night shift workers were similar to those of the control group.