An apatite crystal is a single crystal expressed by a general formula (M2)5(PO4)3X. In this formula, M2 indicates at least one type of element selected from the group consisting of bivalent alkaline-earth metals and Eu, and X indicates at least one type of element or molecule selected from the group consisting of halogen elements and OH. And the single crystal is of a tubular shape. The outer shape of the apatite may be a hexagonal prism. The shape of an opening of a hole formed in the upper surface or lower surface of the hexagonal prism may be a hexagon.