Patients with various cancers benefit from immune checkpoint inhibitors. However, immune checkpoint inhibitor-induced adverse events have also been reported, such as colitis. Prednisolone is the first-line treatment for immune-related adverse events, but second-line therapy for patients refractory to steroids has not been established. Furthermore, the inflammatory cytokine expression pattern in the intestinal mucosa of patients with steroid-refractory immune-related colitis remains unclear. We present the case of a 48-year-old man diagnosed with immune-related colitis and pancreatitis induced by pembrolizumab for advanced lung cancer. First, we administered 50 mg/day of prednisolone, and the patient's abdominal symptoms improved. However, the pancreatic enzyme levels did not return to normal. Furthermore, the patient's diarrhea worsened and hematochezia appeared at a 40 mg/day prednisolone dose. A mucosal cytokine analysis identified a low interleukin-10 messenger RNA level, which has been associated with a poor response to prednisolone. Thus, we administered 5 mg/kg of infliximab; the patient's diarrhea and hematochezia immediately improved, and the pancreatic enzyme levels returned to normal. Infliximab was administered three times every 2 weeks. After, the patient's colitis and pancreatitis did not recur. To our knowledge, this is the first report demonstrating the effectiveness of infliximab for immune-related colitis and pancreatitis.