A prior CRT type oscilloscope can scan electronic beams at extremely high speed in accordance with the input waveform, but a prior matrix type flat display has scanning speed and system limited to individual ones. Thus, in the matrix type flat display, the input waveform is temporarily stored in a memory and required to be read in synchronization with scanning of the matrix type flat display. This prevents the waveform data from being displayed during the writing time. Thus, it will be noted that an image flickers and is therefore observed wtih difficulty.