Between a drilling motor and the upwardly continuing drill string a housing contains a speed sensor rotationally connected to the motor rotor. This hashed value is used to index the table of M locks. Then, the generated image data of the difference small region is coded and outputted, and then the coded image data of the difference small region is decoded, so that the reconstructed image data of the reproduction small region is generated by adding the decoded image data of the difference small region to the image data of the optimum prediction small region.