Craniometry has revealed that continuous skull expansion occurs after dental maturity in macaques and other nonhuman primates. Endocranial volume has been shown to increase with age from mid-adulthood to older age in macaques. Thus, neurocranial thickness may decrease with age, especially from mid-adulthood to older age. Here, we investigated age-related changes in the cranial thickness of Japanese macaques (Macaca fuscata). Ten cranial thickness measurements (ten neurocranial landmarks) were made using computed tomographic scans of 140 crania from adult macaques (67 males and 73 females). The cranial thickness at many sites was shown to increase in the neurocranium from young adulthood (7–9 years) to early-mid adulthood (14–19 years) in males and late- mid adulthood (19–24 years) in females, while it was decreased in the oldest age group (>24 years). The cranial thickness at various sites showed a significant decrease from mid-adulthood to very old age in both sexes, although females had more sites with decreasing thickness than did males. The difference between sexes in terms of age-related changes in cranial thickness at sites on the mid-sagittal plane may be associated with the differences in the size of the projecting face and canines between males and females. The greater number of sites with decreasing thickness in females than in males may be associated with postmenopausal estrogen depletion in female macaques.