This study will examine the efficacy, safety and tolerability of different oral dose of YM060, 5-HT3 receptor antagonist, in patients with d-IBS.

Inclusion Criteria:

• Patient meets Rome II criteria for IBS and the subgroup of diarrhea predominant IBS

Exclusion Criteria:

• Patient with severe cardiovascular disease, respiratory diseases, renal diseases, hepatic diseases, digestive tract diseases (excluding IBS), blood diseases, or neurological or psychiatric diseases
• Patient reported having any Type 1 or Type 2 stool on the Bristol Stool Form Scale during the run-in period