The elevated serum undercarboxylated osteocalcin (ucOC) level is related to osteoporosis. In Japan, vitamin K intake is lower, and the incidence of hip fractures noticeably higher in northern Kyushu than in other areas. The study was done to determine the serum ucOC levels in a Japanese population and its association with diet and glucose metabolism. The data of 3,658 healthy adults aged 40-69 (1,373 men and 2,285 women) who lived in northern Kyushu area were analyzed. The data included anthropometric measurements and a self-reported personal interview on daily intake of foods. The serum ucOC level of each participant was measured by electrochemiluminescence immunoassay. Glycohemoglobin A1c (HbA1c), fasting plasma glucose, and serum insulin concentrations were measured. The median serum ucOC level of the women (4.65 ng/mL) was significantly higher than that of the men (3.04 ng/mL) (P = 0.0021). The age-specific ucOC levels of the men decreased significantly with age. In contrast, the ucOC levels of the women aged ≥50 were elevated, but the levels varied markedly within the other age groups. For both men and women, multivariate analysis identified a daily diet rich in vitamin K and HbA1c level as independently having a significant, negative relationship to serum ucOC level. Our study indicates that the serum ucOC decreases with age in men, increases postmenopausally in women, and correlates inversely with dietary consumption of certain foods and with fasting glucose and HbA1c level.