same therefore even if the same image has been written coordinates whose vertical positions are different are output from the sensing portion control unit 6 fig1 is a flow chart of a program employed in the data processing unit 7 according to the present invention when data from the sensing portion control unit 6 is received a fact that which one of the writing surface of the frame 15 faces the surface of this drawing sheet is detected on the basis of the outputs from the non contact sensors 95 and 96 step s7 and the vertical coordinate for example y coordinate is used intact or is converted into a value obtained by subtracting this coordinate from the maximal value in the subject direction step s8 therefore according to the present invention both of the writing surfaces can be used in the same manner regardless of consciousness of recognizing the writing surface according to this embodiment the metallic member 94 and the non contact sensor 95 and 96 form means for detecting the writing surface which is being used while the program and microprocessor 71 shown in fig1 form coordinate conversion means the sensing portion described in the first and second embodiments is usually formed by an insulating substrate having a printed conductive pattern as to correspond to the positions of the above described x and y direction loop coils alternatively a structure may be employed which is arranged such that a member in which a multiplicity of conductive wires are at predetermined intervals held between two insulating films.