[Purpose] This study examined physical factors associated with urinary incontinence (UI) in women. We hypothesized that, women with UI would show decreased thickness of the transverse abdominal muscle (TA) during maximal co-contraction of both TA and the pelvic floor muscle (PFM) compared with the women with no history of UI. [Subjects] The subjects were thirty-two women who subjects were divided into two groups: the UI group and the No-UI group. [Methods] We evaluated the thickness of TA by ultrasound, and hand-grip strength and the muscular strength of adduction in flexion of hip joint. The thickness of TA was measured during performance of 4 tasks: (1) at rest, (2) maximal contraction of TA, (3) maximal contraction of PFM, and (4) maximal co-contraction of both TA and PFM. [Results] The No-UI group had thicknesses of TA that were greater than those seen in the UI group in the tasks 3 and 4. In logistic regression analysis with UI as the dependent variable, the thickness of TA during maximal co-contraction was identified as an independent factor, and the cut-off value of the thickness of TA was 2.55 mm as determined by the Receiver-Operating-Characteristic (ROC) curve. [Conclusion] We found that the thickness of TA during maximal co-contraction of both TA and PFM is reliable and useful for the evaluating the risk of UI in women.