PURPOSE: To form the sampling pulse synchronous to the peak of a pulse wave signal with high accuracy, by separately detecting the peak part of a pulse wave signal by a threshold value variable comparator and a differentiation circuit and applying AND operation thereto.

CONSTITUTION: A signal containing both of pulse wave data and pressure data of blood pressure is obtained by a pressure sensor 1 and subsequently amplified by a differential amplifier 2 to output a signal (a). This signal (a) becomes a pulse wave signal (b) by removing the DC component thereof to be respectively inputted to a threshold value variable type comparator 9, a differentiation circuit 7 and a sample holding circuit 11. The pulse wave signal (b) inputted to the comparator 9 is compared with threshold voltage VTH1 becoming reference and the pulse corresponding to the peak part of said pulse wave signal (b) is outputted by the comparator 9. The pulse wave signal (b) inputted to the differentiation circuit 7 is differentiated and a signal (c) having the peak value of the signal (b) as a cross point is outputted from the differentiation circuit 7. The AND of the signal (c) and a pulse signal (d) is taken by an NAND circuit 10 performing AND operation to output a pulse having a fine pulse width.