PURPOSE: To perform waveform recording with fidelity, by using a representative value, which is determined with respect to a basic time interval immediately before, as a reference value for determining the representative value of the basic time interval, which is synchronized with the timing of recording.

CONSTITUTION: In the first channel (b), an input signal (a) is processed by a driving circuit 11 and imparted to a recording pen 12. In the second channel (c), an input signal is sampled by a sampling means 24 and digitized by an A/D converter 231. The representative value is determined by using a reference value from a reference value memory 234 in a representative-value determining circuit 232. The representative value is stored in a representative-value memory 233. After the deviation of the pen position is delayed, the representative value is converted into the analog value by a D/A converter 235 and recorded by a recording pen 22 through a recording-pen driving circuit 21. A part of the representative value obtained at this time is sent to the reference value memory 234 and adopted as the reference value for determining the representative value in the next basic time interval.