A total of 49 patients were enrolled in this study. Voiding symptoms were investigated on medical records at 3 and 6 months after retropubic retrograde radical prostatectomy. Incontinence was defined as any involuntary urine leakage. First, clinical and surgical factors were analyzed in continent and incontinent patients. Then, the thickness of the pelvic diaphragm and the levator ani of 38 patients were measured on a monitor using a preoperative magnetic resonance image. The differences between these factors and the continent status were analyzed. Incontinence was noted in 57 and 29% of the patients at 3 and 6 months, respectively. There were significant differences in the thickness of the pelvic diaphragm on sagittal images between the continent and incontinent groups at both 3 and 6 months postoperatively (p = 0.006 and p = 0.02, respectively). There were no differences on other factors. The pelvic diaphragm thickness may be a predictive factor of post-prostatectomy incontinence. The data suggested that patients having a thin pelvic diaphragm might not be expected to achieve earlier recovery of continence after radical prostatectomy.