It has been said that the innermost enamel of erupted human teeth is either a hyper- or a hypomineralized layer. Using SEM-EDX, in the present study, the Ca and P mineral contents of the innermost 10-15 microns enamel showing a highly negative birefringence (IE-I) were analyzed and compared with those of the adjacent inner enamel (IE-II) in each cuspal-third, mid-coronal, and cervical-third region. The Ca content in the IE-I zone was significantly lower than that in the IE-II zone in the mid-coronal and cervical-third regions, whereas the significant difference of P content was seen only in the cervical-third region. In the other areas, the IE-I zone tended to be lower than the IE-II zone. These significant and no significant differences of Ca and P contents may be due to the difference in the amount of organic substance in the IE-II zone of the three regions, and that of these mineralization rates after eruption of a tooth.