Defects in the insulator bushing are the major caution of destroying a switchgear. An epoxy bushing is composed of epoxy-molded insulator layer and a conductor. That means, a porosity or delamination defect could be included in the insulating layer by the manufacturing process. An inspection method is required to secure integrity of the bushing. An ultrasonic-immersion system has the power to produce a required effect to examine critically an epoxy material with high degree of fineness. In this research, an optimized ultrasonic immersion system was developed and applied to examine critically the epoxy-layer of bushings. As results of the result of a careful examination, both artificial defects and delamination were detected by the system. Currently, the ultrasonic-immersion system should be applied for examining the epoxy-layer of the bushing carefully.