PURPOSE: To compensate the span fluctuation with high accuracy over a wide range of the exciting frequency and without using any RC delay circuit, by using a microcomputer for the signal process and compensating the span fluctuation with an operation.

CONSTITUTION: The span compensation degree α due to the type of a transmitter based on its caliber, etc. and the exciting frequency is obtained previously and stored in an ROM12. Then a CPU11 reads the degree α out of the ROM12 based on the designation of selection for the caliber of the transmitter via a setting switch 16 and the frequency of a power supply 8 which is measured by a frequency counter 17. Thus the correct flow rate value is obtained from V0= Vs/α to the measured flow rate value Vs. At the same time, the frequency (f) of the using mode is counted by the counter 17 and stored in an ROM14. The selection of the transmitter caliber is designated by the switch 16, and the corresponding compensation data β and γ are read out of the ROM12. Then the CPU11 calculates the correct flow rate value V0 from V0=Va/[β+γ(f-f0)]. This result can be converted into an analog signal through a D/A converter 13 and then delivered.