The belief in a just world can have an influence on the learning success of students because students who believe in a just world are willing to invest more time in learning. Previous studies have already shown that people who believe in a just world achieve better grades. However, measuring learning success using a single indicator does not provide sufficient information, because learning success depends on many factors (e.g. prior knowledge). For this reason, the influence of belief in a just world on objective and subjective knowledge changes is measured for a game-based learning application in this study with an online survey. The results of the study show that the students achieve subjective and objective knowledge gains. Nevertheless, not many significant correlations could be identified between the students’ belief in a just world and objective or subjective knowledge changes. These results contradict previous studies that reduced the measurement of learning success to a single indicator.