The bone is composed of the bone matrix of collagen and hydroxyapatite, the mixture of calcium and phosphorus. The bone tissue is considered to the special connective tissue that possesses extracellular matrix made by collagen fiber deposited with mineral complex. In order to maintain bone mass measured by the sum of bone matrix and hydroxyapatite, bone resorption by osteoclast during lifetime and bone remodeling to form bone by osteoblast in its resoprtion region repeat continuously. The osteoblast has a mesodermic fetal origin like fibroblast for the formation of form tissues. Two cells express identical genes and synthesize the identical collagen type Ⅰ as the major component of the formation of bone matrix and skin. Therefore, it is considered that the decrease of skinfold thickness and the decrease of bone mass related to the age, the change of two tissues composed of collagen type Ⅰ is caused by the same genetic mechanism. The decrease of bone mass is caused by the change of the amount and structure of bone matrix by several factors and the amount of minerals deposited on bone matrix. Especially, in case of female, the deficiency of estrogen by menopause makes these changes rapidly increased. The decrease of bone mass and skinfold thickness is due to the decrease of the amount of collagen and its structural change the common component of bone tissue and skin tissue. Therefore, the relationship of the amount of bone collagen, bone mass and skinfold thickness may be examined by determination of the amount of cross-linked peptide N-telopeptide, collagen metabolite which excretes as urine. Based upon the proved results about the significant relationship of bone mass, the amount of bone collagen, the amount of skin collagen and skinfold thickness, the bone mass may be expected through a facile determination of skinfold thickness.