Abstract: This invention relates to an oscilloscope in which waveforms can be indicated on a matrix type plane display such as a liquid-crystal display or plasma display as well as on a CRT (cathode-ray tube). The oscilloscope of the invention comprises a memory in which waveform data can be written or from which it can be read, a writing control circuit to control writing the waveform data on the memory, a reading control circuit to control reading the waveform data from the memory and a display to indicate the read waveform data. The writing and reading control circuits have a predetermined writing prohibition period during which writing is prohibited after writing all the data of one waveform. The writing period during which data of one waveform is written has a minimum uniform length, and a period during which the data can be availably read from the memory becomes longer as the writing frequency becomes longer.