1. A handwriting input/output system comprising: a medium that comprises a writing area, on at least part of or a plurality of areas of the medium, formed with a dot pattern that repeatedly defines coordinate information and code information; an imaging unit that images the dot pattern of the writing area; and a dot pattern analysis unit that analyzes a code of the image data of the dot pattern imaged from the imaging unit to obtain the code information and trajectory information created by the coordinate information; wherein the medium comprises a plurality of the writing areas, in at least a part of the code information defined in each of the plurality of the writing areas, a specific code which uniquely specifies the writing area is defined.