Here, we report about a woman in her 30s who had peritoneal dissemination and multiple colon cancer with high-frequency microsatellite instability(MSI-H). Her father, paternal grandfather, and maternal grandmother had a history of colorectal cancer treatment. Thus, Lynch syndrome was suspected. We performed R0 resection for peritoneal dissemination and subsequent peritoneal dissemination. A 435-gene panel testing using a next-generation sequencer identified MSH2 and other mutations in the tumor. Hence, we speculated that she could have a germline mutation of MSH2, which causes Lynch syndrome. In the future, if she wishes to receive genetic counseling and undergo germline testing for variants to confirm the diagnosis of Lynch syndrome, we will perform them after receiving informed consent.