The purpose of this study was to investigate the correlation between problem-solving abilities, self-efficacy for group work, and affective achievements and examine the mediating effect of self-efficacy for group work in the relationship between problem-solving abilities and affective achievements targeting Meister high schools which conduct project-based learning. Analyzing 160 Meister High School students using SPSS and Process Macro Model 4, the result shows that the dependent variable, which is affective achievement, the independent variable, which is the problem-solving ability (problem-solving confidence and approach-avoidance style), and the mediating variable, which is self-efficacy for group work, show a significant correlation. In addition, self-efficacy for group work is completely mediated between problem-solving abilities and affective achievement, and sub factors of problem-solving abilities which are problem-solving confidence and approach-avoidance style are as well. Based on the results, it is found that self-efficacy for group work is a very important factor for successful project-based learning. And then, implications from the teaching methodological side are suggested.