Background. Instruments for measuring the presence and severity of specific irritable bowel syndrome (IBS) symptoms, comparable to those used in Western countries, have been lacking in Japan. The aim of this study was to develop, validate, and confirm the reliability of the Japanese version of the Rome Ⅱ modular questionnaire for IBS (RⅡMQ-J) and the IBS severity index (IBSSI-J). Methods. Forty-nine patients in the university hospital with chronic or recurrent abdominal pain and discomfort and/or altered bowel habits were enrolled. With Rome Ⅱ criteria, 27 patients were diagnosed as having IBS, and the other 22 patients were evaluated as having other functional bowel disorders (FBDs). The English versions of RⅡ MQ and IBS SI were translated into Japanese. After back-translation and approval of the questionnaire, subjects completed both questionnaires twice within 14 days. Results. Cronbach’s alpha of the RⅡ MQ-J was high (0.72). The sensitivity of RⅡ MQ-J for the diagnosis of IBS was also high (89%). The specificity of RⅡ MQ-J for denial of IBS among patients with other FBD was satisfactory (73%). The IBS SIJ showed high internal consistency (0.69) and reproducibility (intraclass correlation coefficient, 0.86, P < 0.001). Conclusions. The RⅡ MQ-J and IBS SI-J are valid, reliable, and appropriate instruments for detecting and assessing the severity of IBS status in Japanese patients. Background. Instruments for measuring the presence and severity of specific irritable bowel syndrome (IBS) symptoms, comparable to those used in Western countries, have been lacking in Japan. The aim of this study was to develop, validate, and confirm the reliability of the Japanese version of the Rome II modular questionnaire for IBS (RIIMQ-J) and the IBS severity index (IBSSI-J). Methods. Forty-nine patients in the university hospital with chronic or recurrent abdominal pain and discomfort and / or altered bowel habits were enrolled. With Rome Ⅱ criteria, 27 patients were diagnosed with IBS, and the other 22 patients were as having other functional bowel disorders (FBDs). The English versions of RⅡ MQ and IBS SI were translated into Japanese. After back- translation and approval of the questionnaire, subjects completed both requests for twice within 14 days. Results. Cronbach’s alpha of the RII MQ-J was high (0.72). The sensitivity of RII MQ-J for the diagnosis The specificity of RII MQ-J for denial of IBS among patients with other FBD was satisfactory (73%). The IBS SIJ showed high internal consistency (0.69) and reproducibility (intraclass correlation coefficient, 0.86, P <0.001). Conclusions. The RII MQ-J and IBS SI-J are valid, reliable, and appropriate instruments for detecting and assessing the severity of IBS status in Japanese patients.