To demonstrate the superiority of YM060 to a placebo in global assessment of relief of overall IBS symptoms and stool form normalization, and to evaluate safety in female patients with diarrhea-predominant irritable bowel syndrome (D-IBS).

Inclusion Criteria:

1. Patients satisfying the Rome III Diagnostic Criteria
2. Patients whose ≥25% of stools were loose or watery and <25% of them were hard or lumpy stools
3. Patients who have abdominal pain or discomfort
4. Patients in whom after occurrence of IBS symptoms the following tests were conducted and in whom no organic changes were observed Pancolonoscopy or contrast enema

Exclusion Criteria:

1. Patients with a history of surgical resection of the stomach, gallbladder, small intestine or large intestine
2. Patients with a history or current evidence of inflammatory bowel disease
3. Patients with a history or current evidence of colitis ischemic
4. Patients with concurrent infectious enteritis
5. Patients with concurrent hyperthyroidism or hypothyroidism
6. Patients with concurrent active peptic ulcer
7. Patients with other concurrent disease that may affect the digestive tract passage or large intestinal function or that is likely to interfere with proper assessment of IBS abdominal pain/discomfort