The purpose of this study is to investigate the effect of PBL (Problem-Based Learning) on ‘key competencies’ in a welding practice of a specialized high school. In order to achieve the purpose of this study, a practical class with PBL was developed and applied to students in a welding practice of a specialized high school facility department, and a basic vocational competency test presented in the related prior studies was examined. First, the welding practice with the industrial facility department was analyzed, and the study material and draft for the teaching·learning process to which the PBL was applied were developed, and then corrected and supplemented through expert verification. Next, the industrial facility department students in C industrial specialized high school located in Daejeon were divided into 19 experimental and control groups, respectively, and PBL was applied to the experimental group and traditional learning methods were applied to the control group. Pre/post-tests were conducted to the experimental group and the control group before and after class as a vocational key competency test sheet, and the results were analyzed using the corresponding sample t-test. The results of the study through the above process are as follows. The basic vocational competency of the experimental group improved significantly, and that of the control group did not show any significant change. Therefore, it has been empirically confirmed that PBL has positive effects on the improvement of students’ basic vocational competency in industrial facility department welding practice.