Mushrooms that had been handpicked were butter-roasted and eaten by six neighborhood residents. All 6 individuals subsequently developed nausea within 60 min to 90 min after the meal. They requested an ambulance and were brought to our emergency department. The ambulance service staff mentioned the possibility that the ingested mushrooms were Lampteromyces japonicus. After admittance, symptomatic conservative treatment relieved the symptoms and all 6 individuals were discharged the next day. However, one patient was rehospitalized due to complaints of abdominal pain and anorexia and another patient was rehospitalized with complaints of vomiting and bloody stool 2 days after initial discharge. In the latter patient, abdominal computed tomography revealed marked wall thickening of the duodenum and jejunum. Typically, the symptoms caused by Lampteromyces japonicus are vomiting, diarrhea, and abdominal pain within 30 min to 3 h after ingestion, but intestinal edema may occur after a few days in severe cases.