Although aging is well established as an important risk factor for aortic stenosis, the mechanism of age-related aortic valve calcification is yet unknown. Here, we investigated this mechanism in tissue and cellular levels using middle-aged rats. Aortic valve specimens were obtained by dissecting from 9-week-old (young) and 30-week-old (aged) male Wistar rats. In the aged rats, the main risk factors for aortic stenosis in plasma were still in the normal range; however, their number of calcified specimens was significantly increased in comparison with the young rats. Aortic valve interstitial cells (AVICs) obtained from explants of aortic valve specimens were cultured for 14 days after reaching confluence. Spontaneous calcification, the expressions of calcigenic genes, that is, BMP-2, alkaline phosphatase (ALP), and osterix (osteogenic transcription factor) and ALP enzyme activity in AVICs from aged rats were enhanced in comparison with those from young rats. However, neither typical calcification inducing reagents (dexamethasone, β-glycerophosphate, and high concentration of phosphate) nor tumor necrosis factor-α (an inflammatory cytokine) accelerated the spontaneous calcification of AVICs from aged rats. These results suggest that aortic valve calcification progresses with age partly through an activation of the BMP-2 pathway.