A direct current and an alternate current are simultaneously supplied from a
constant
current circuit to a detection coil. At least one of the ratio of the direct current
to the alternate current, the ratio of an alternate current component to a direct
current component of an impedance in a detecting section, a temperature characteristic
of the ratio of the direct current to the alternate current, and a temperature
characteristic of the ratio of the alternate current component to the direct current
component of the impedance of the detecting section is set in such a manner that
the margin of fluctuation of a temperature coefficient of a peak value of an output
voltage from the detecting section in a whole displacement zone of a core relative
to the detection coil is smaller than the margin of fluctuation of a temperature
coefficient of the alternate current component of the impedance of the detection
coil at the frequency of the alternate current.
