PURPOSE: To reduce noise and to improve SN ratio at high exciting frequency while ensuring the sampling interval without modifying the exciting coil by switching the connection of exciting coil in series or parallel.

CONSTITUTION: A switch 12 switches the connection of a coil 4 in series and parallel while generating a magnetic field in same direction. When the coil 4 is excited with higher frequency, the impedance of the coil 4 increases in proportion to the exciting frequency but when the connection of the coil 4 is switched from series to parallel by means of the switch 12, the exciting current of the coil 4 can be decreased to about one half of normal level. Consequently, the magnetic field generated by the coil 4 is restrained from decreasing and a relatively stable signal electromotive force is produced thus improving the SN ratio relevant to the fluid noise by an amount corresponding to the increase of exciting frequency. Furthermore, the rising time of signal electromotive force is restricted even in case of high exciting frequency and a sufficient sampling interval is ensured.