Detection error resulting from rain, in the case of clear weather, and in
the case of rainy weather are included in measurements by a measurement
unit disposed in an MW sensor system. The error resulting from rain
sharply increases as a preset .DELTA.f becomes larger. The error in the
case of clear weather sharply decreases as the preset .DELTA.f becomes
larger. Using .DELTA.f as a parameter, a range, in which the rate of
variability of the detection error resulting from rain with respect to
.DELTA.f is low overlaps with a range where the rate of variability of
the detection error in the case of clear weather is low is set as the
range of the detection error in the case of rainy weather. The detection
error in the case of rainy weather becomes lower than the value of the
detection error in the case of clear weather.