In the sodium-cooled fast reactor (SFR), conventional optical visual inspections cannot be applied due to the opacity of sodium coolant. An inspection technique based on the ultrasound should be employed because the opacity of sodium prevents any conventional techniques using optical devices from being applied for under-sodium viewing of SFR [1]. Recently, the 10 m long plate-type ultrasonic waveguide sensor has been developed for the under-sodium viewing of SFR [2]. In the plate-type ultrasonic waveguide sensor, an ultrasonic transducer installs above reactor head which is in a relatively cold condition, A0 mode Lamb wave is propagated in a plate waveguide and an ultrasonic beam is radiated into hot radioactive liquid sodium. Its imaging capability was successfully demonstrated through under-sodium experiments [3]. To enable ongoing operation, the SFR requires refueling and this refueling process is much more difficult than that of a pressurized-water reactor (PWR) because the SFR uses sodium as its coolant. Sodium reacts with air or water and thus the refueling in the SFR is conducted by rotating the rotation plugs in the reactor head without opening the reactor head. And to ensure the safety of refueling process, a ranging inspection that monitors the presence of any obstacles (control rods, fuel sub-assemblies, etc.) between the reactor core and the upper internal structure, which can disturb the rotation of the rotation plugs, is essential. In this work, the plate-type ultrasonic waveguide sensor that can radiate a horizontal ultrasonic beam has been newly designed for the ranging inspection of SFR. A beam profile measurement test was carried out first to investigate the characteristics of the developed sensor. And then several under-water tests in the mockup facility using the developed plate-type ultrasonic waveguide sensor were conducted to evaluate the feasibility for ranging inspection.