Using the biomimetic method, we formed a hydroxyapatite (HAp) layer on/in certain types of nonionic hydrogels that contain hydroxyl groups. The hydrogels used were poly(vinyl alcohol) (PVA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(glucosyloxyethyl methacrylate) (PGEMA), and agarose. Under an optical microscope, we observed a thin, continuous HAp layer on the top surface of the PVA, PHEMA, and PGEMA gels. On the other hand, we only observed an intermittent HAp layer on the surface of the agarose gel. The swelling ratio and the bound water content of these hydrogels were measured as an essential character in HAp formation. There was some relation among the HAp formation, the swelling ratios, and the bound water content.