Resistance spot welding has been widely used for the construction of thin sheet metal structures. The structures’ reliability depends upon the weld quality as well as the weld size. Therefore, various non-destructive inspections have been applied to the spot welds. Among them, the ultrasonic evaluation is widely used for the detection of defects and for the measurement of weld size. However, this method is too difficult for the detection of the fusion zone or nugget non-destructively, in spite of the high demand for their evaluation. Due in part to the recent application of high-strength steel sheets to automobiles, this problem has become a crucial subject in spot welding. The discrimination of the nugget from the solid state-welded zone or the corona bond is now strongly demanded. This study focused on the non-destructive discrimination between the nugget and the corona bond; a new, ultrasonic measurement with a line-focused probe was proposed. In the experiment, spot welded specimens both with and without surface indentations were prepared, in order to determine the affecting factors on the ultrasonic measurement of spot welds. Moreover, two types of special, thin sheet specimens were prepared to determine the acoustic phenomena in the fusion zone. The first type was the resistance spot-welded specimen and the other was the gas tungsten arc-welded specimen. The measurement of these two special specimens showed that the sound velocity in the nugget differed from the base material and was dependent upon the inhomogeneous microstructures. The different sound velocity caused a different amplitude of reflected waves in the nugget, the change of which also depended upon the measuring conditions of the water path. From these results, it was shown that the measurement based on the different amplitude and the dependence on the water path enabled the non-destructive discrimination between the nugget and the corona bond.