PURPOSE: To display waveform data large in capacity without dissipating information providing in them by obtaining the luminance of an interpolation line based on a difference between inputted waveform data, and accumulating the luminance of the interpolation line in a correspondent picture element in image memory.

CONSTITUTION: Waveform memory 10 outputs the waveform data Vb for display sequentially, and a DFF 12 outputs the waveform data Va one before the data Vb. A substractor 11 calculates the difference of data Vd=Vb-Va. The differ ence Vd is inputted to a ROM 14, and the luminance Sb of the interpolation line is calculated. Thereby, a part with high (low) through rate in a waveform can be displayed darkly (brightly). An interpolation line generator 13 generates a vertical address Av from the data Vb to the data Va sequentially, and it is inputted to the image memory 16 with a horizontal address Ah generated by a horizontal counter 17. In such a way, the luminance can be accumulated by making access all the picture elements of the interpolation line sequentially, and increasing the luminance of each picture element by the luminance of the interpolation line with a latch 18 and an adder 15. After one interpolation line is plotted, the memory 10 outputs the next waveform data Vb.