An apatite layer was formed on a transparent polystyrene (PS) substrate using a simple surface modification process, in which apatite precursors were deposited on a PS substrate surface that was then immersed in a calcium phosphate solution supersaturated with respect to apatite. The apatite-coated PS substrate thus obtained was highly transparent, which enables in situ observation of cell behavior on the surface apatite layer under a transmission optical microscope.