PURPOSE: To record a display image at an optional point of time while observing a waveform continuously at all times by providing a waveform memory for storing waveform data to be displayed and a frame memory for storing image data outputted to a display part.

CONSTITUTION: A display control circuit 2 reads waveform data SD stored in the waveform memory 1 in order and converts them into serial type image data SV. Then the image data SV while outputted to a CRT 3 to display waveforms are outputted to a serial-parallel converter 6. The serial-parallel converter 6 converts them into parallel type image data SV at an optional point of time and stores them in the frame memory 7. The stored parallel image data SV are read out and recorded on a recording device 5 according to a readout control signal SC4 applied by a recording control circuit 4. Consequently, while the waveforms are observed at all times, display images at optional points of time can be recorded.