PROBLEM TO BE SOLVED: To provide technology for calculating the distance with high accuracy regardless of receiving intensity even if a distortion of a received waveform is caused by STC. SOLUTION: In the case where the amplification factor of the STC circuit changes with the time, as is known from comparison between waveforms 1 and 2 in (c), (d), both the rise and fall are delayed by STC waveform distortion as compared with the case of no distortion, so the middle time of the crossing time with a threshold is delayed as compared with the case of no STC distortion. In the case of an error in middle time, it comes to an error corresponding to (an error of the rise time (a part indicated by 2) + an error of fall time)/2. It is necessary to correct the error for that, and the degree of STC waveform distortion, as is known from the comparison between (c) and (d), varies with intensity difference of the received signal as well. Accordingly, the correction time is previously obtained by experiments or the like in consideration of the point that the amplification factor changes with the time, and the point that the influence of STC distortion changes with the signal intensity, and a measurement error is corrected by the correction time. COPYRIGHT: (C)2003,JPO