An advanced small bowel mucinous adenocarcinoma with Peutz-Jeghers syndrome was resected, and we started capecitabine plus oxaliplatin (CapeOX) as adjuvant therapy. However, local recurrence was noted, and the tumor increased even after CapeOX plus bevacizumab and fluorouracil plus leucovorin plus irinotecan plus panitumumab (FOLFIRI plus panitumumab). Pembrolizumab was administered after confirming high-frequency microsatellite instability, and the tumor shrank markedly and remained shrunk for 20 months.