PURPOSE: To prevent the observation error by measuring first and second intensities of rainfall or the like at the time of changing the angle of elevation of a radio beam to throw this beam and determining the distance to a drop point in accordance with the height difference between the drift distance obtained from a reference point and each observation height.

CONSTITUTION: Two formulas lc=lA+Δl.H1/(H1-H2) and Δl=lb-lA are true where H2, H1, lA, lB, Δl, and lC are the low (first) observation height which is set at the time of emitting the radio beam of a parabolic antenna in the horizontal direction, the high (second) observation height which is set at the time of changing the angle of elevation of the parabolic antenna to emit the radio beam to the direction higher than the horizontal direction, the distance from the reference point of the parabolic antenna to a ground point just under a rain drop 8 at the height H1 in the perpendicular direction, the distance from a reference point 0 of a radar to a ground point just under the raindrop 8 at the height H2 in the perpendicular direction, the distance of drift of the raindrop 8 due to wind, and the distance to the drop point of the raindrop 8 whose intensity of rainfall is measured respectively. Though the obtained distance lC of the raindrop 8 is determined as the actual drop point of the rainfall, this determination is true only when there is a difference between first and second rainfall intensities.