PURPOSE: It is theorized that the prevalence of obesity has not decreased owing to poor adherence to implemented programs addressing metabolic syndrome, obesity, and diabetes in Japan. Therefore, we intended to evaluate the influence of a moderately low-carbohydrate diet on improving markers of metabolic syndrome among workers in Japan. PATIENTS AND METHODS: Participants with metabolic syndrome or obesity were recruited based on the eligibility criteria for the Specific Health Guidance program and educated on a moderately low-carbohydrate diet between spring 2016 and fall 2018. The participants were then made to report their food intake and body weight once a week for the next 12 weeks and were counselled on maintaining a moderately low-carbohydrate diet. HbA1c levels, lipid profile, body weight, and sleep quality were evaluated. The normality of the data was evaluated using the Skewness/Kurtosis test. Each variable was compared before and after the intervention using the Wilcoxon signed-rank test. Further, a subgroup analysis of the data from the participants whose variables were abnormal at baseline was performed. RESULTS: Among the 101 enrolled participants, a decrease in the median weight (from 82.5 to 79.7 kg, p<0.001, n=46), body mass index (from 27.3 to 26.9 kg/m2, p<0.001, n=46), and apnea-hypopnea index (from 24.1 to 17.1, p<0.01, n=39) was observed. Subgroup analysis of participants with abnormal baseline values revealed changes in HbA1c (from 6.7% to 5.8%, p<0.001, n=34), total cholesterol (from 220 to 209 mg/dL, p<0.01, n=54), low-density lipoprotein cholesterol (from 133 to 120 mg/dL, p<0.001, n=31), high-density lipoprotein cholesterol (from 35 to 40 mg/dL, p<0.01, n=31), triglycerides (from 242 to 190 mg/dL, p<0.01, n=57), and deep sleep percentage (from 10.4% to 18.2%, p<0.05, n=7). CONCLUSION: A moderately low-carbohydrate diet may be considered a potential intervention for improving the markers of metabolic syndrome, obesity, and diabetes.