To make output errors generated due to changes in power source voltage small without being accompanied with increase in cost and to prevent such a state as being reduced in versatility.

A signal voltage Va of an absolute value level from a piezoelectric element 5 is given to an absolute voltage-ratio conversion circuit 10 through an amplification circuit 7, a filter circuit 8 and a signal correction circuit 9. The absolute voltage-ratio conversion circuit 10 converts a differential voltage between the inputted signal voltage and an absolute reference voltage VFER, which does not depend on a power source voltage to be outputted from an absolute reference voltage circuit 4 to a signal voltage, with the level thereof proportional to that of a voltage for comparison proportional to the power source voltage and gives the converted signal voltage to an A/D conversion circuit 11 provided on the system side. Voltage for comparison is obtained as the difference between the partial voltage from a voltage dividing circuit 2 for dividing the power source voltage and the partial voltage from the auxiliary voltage dividing circuit, provided inside the absolute voltage-ratio conversion circuit 10, to divide the power source voltage at a division rate differing from the voltage dividing circuit 2.