a peak detecting threshold determining method is provided which determines a peak detecting threshold which is used by an fmcw radar in detecting a peak frequency component which appears as representing a target object in a frequency spectrum . a cw radar wave is transmitted to produce a cw noise spectrum . an offset is added to frequency components in a high - frequency region of the cw noise spectrum to define a first distribution as a value of the peak detecting threshold . this enables the value of the peak detecting threshold in the high - frequency region to be determined with high precision by reflecting a receiver noise containing a leakage noise .