PURPOSE: To judge the contents of input digital data while grasping its variation with time by forming a boundary mark at a timing position where the input digital data varies and displaying the input digital data almost at the center position of a section specified with marks.

CONSTITUTION: The input data in binary code is converted by a code converting circuit 11 into data a hexadecimal code, and a counting circuit 12 for the number of continuation detects the timing where the digital data varies and also counts the number of the same digital data present in a section formed by the timing. Then, a frame generating circuit 13 generates the boundary line of the position corresponding to the timing of the variation of the digital data and the number of connection lines is determined in correspondence to the number of the same continuous digital data to determine the size of a frame. The display position of the data in hexedecimal code is displayed by a code display circuit 14 almost at the center position of the sectioned formed by boundary lines, so the visibility is improved.