Students' difficulties in differentiating the direct proportion and inverse proportion problems cause interference. Proactive interference is the error that occurs when old information (concept of direct proportion) interferes with new information (concept of inverse proportion). In solving the problem of inverse proportion, students often use the concept of direct proportion. The student's mental structure regarding the concept of proportion as a result of previous learning is referred to as met-before. Therefore, this study aims to describe the met-before of students who experience proactive interference. This research is a case study involving 32 8th-grade students in Malang, Indonesia. These subjects were students who experienced proactive interference with specific fluency of communication and willingness. Data was collected through proportion problems and interviews. Students' work was analyzed based on the description of the met-before. The results showed that students who experienced proactive interference with the non-flexible type had suppressed problematic, while students with the flexible type have focus supportive met-before in solving direct proportion problems. Both students with non-flexible type and flexible type have focus problematic met-before when solving inverse proportion problems. This is because met-before about cross multiplication strategy interferes with students' problem-solving.