PURPOSE: To precisely measure respiratory waves, accordingly the ventilation amount, through electrocardiographic signals by calculating the direction of the absolute sum type electricity axis at every heart beat on the basis of the absolute values of amplitudes sensed, and preparing an approximation curve by means of plotting each piece of data obtained through calculations from time to time.

CONSTITUTION: An electrocardiographic signal sensing circuit 1 emits an electrocardiometric signal of the induction in the direction perpendicular to a heart vector or of the induction of a certain angle which is convertible into the orthogonal axis. From the QRS waves in the electrocardiographic signals, an electrocardiographic signal analyzing means 2 senses the R-wave amplitude AR1 and S-wave amplitude AS1 constituting the abscissa axis component of the mean spectrum and the R-wave amplitude AR2 and S-wave amplitude AS2 constituting the corresponding ordinate axis component. On the basis of the absolute values of the sensed amplitudes AR1, AS1, AR2, AS2, a calculating means 3 calculates the direction θ of the absolute sum type electricity axis at every heart beat from the formula θ=tan-1 {(AR2 + A2)/(AR1 + AS1)}. A respiratory wave forming means 4 prepares the approximation change curve by plotting Bs, and an output means 5 displays or records the obtained respiratory wave curve.