PURPOSE: To reduce the measuring error by deflecting or absorbing the electric waves arriving in multiple reflection peripheral or near the receiving antenna and thus preventing the multiple reflected waves sent from the subject from going into the antenna.

CONSTITUTION: Electric wave W radiated from point A of transmitting antenna ANT is reflected at point B owing to the inclination of water surface S to be electric wave WO (necessary signal wave) going to point A, electric wave W1 going to point C1 of reflecting plate REF and electric wave W2 going to point C2 each (W1, W2: noise causes). The antenna receives only wave WO which comes back directly, and thus the measuring error is eliminated or reduced considerably. To secure in the more effective and assured way the elimination of the multiple reflected waves arriving onto effective surface TR inevitably due to the scattering or the diffused reflection, etc., the electric wave absorbing material is coated over the surface of the electric wave sensitive part on reflective surface REF. And thus the electric waves can be absorbed at the areas excepting parabola surface PA.