It is an object of the present invention to provide a biochemical instrument wherein an adhesive layer that has hydrophilic properties, heat resistance, and satisfactory adhesiveness is provided between a plastic material surface and a hydrophilic polymer layer. The present invention provides a biochemical instrument comprising a plastic material that is coated on its surface with a hydrophilic polymer, which comprises a metal oxide thin film between the plastic material and the hydrophilic polymer layer, wherein the film stress of said metal oxide thin film is less than 5.4.times.10.sup.21 dyn/cm.sup.2.