An experimental study is carried out to clarify the performance of an eddy current testing probe and probe-inserting equipment for the in-service inspection of the intermediate heat exchanger tubes of the High-Temperature Engineering Test Reactor. Artificial discontinuities are made with reference to the American Society of Mechanical Engineers standards for steam generator tubes in a light water reactor. It is confirmed that the probe can detect these discontinuities as well as smaller ones, such as a 0.5-mm-diam 100% through-wall hole and a 0.5-mm-wide groove, in a base-metal tube. For the welded joints, the back-excess weld metal is a main noise contributor, and a multiple-frequency method can remove the noise. The inspection performance, however, is lower. The probe-inserting equipment can smoothly insert and extract the probe. The winding of the cable causes a scattering in the probe traveling velocity values and a measurement error regarding the probe’s location in the tube.