Air bubbles or foreign objects in a transparent adhesive layer that bonds a transparent plate and a plate-like member together in full-surface contact are detected without fail. For the detection, an optical fiber or the like is used to irradiate the adhesive layer with light from a side, the cast light is guided through the adhesive layer, and a state of the adhesive layer is checked with the guided light. A check that discriminates defects in the adhesive layer from defects in the bonded transparent plate is thus made possible.