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.