A five-year-old male Shih-Tzu dog presented with severe vomiting and weight loss. The clinical signs were successfully improved by an eight-day treatment with an H2-receptor antagonist, gastrointestinal protectant and antibiotics. Ten days later, however, recurrence of vomiting was seen despite continuous medical treatment. Based on clinical signs and the results of various diagnostic tests including CBC, biochemical analysis, contrast radiography, and endoscopy, a duodenal or pancreatic neoplasm was suspected and exploratory laparotomy was conducted. Some swollen pancreatic regions were found, and biopsy of the pancreas indicated the diagnosis of a gastrin-secreting tumor. Consequently, based on a high serum gastrin level as well as clinical signs and immunohistological findings, we diagnosed the disease as canine gastrinoma, a rare tumor of the pancreas.