A rare case of urogenital sinus cyst in a child is reported. A 10-month-old boy was referred to our hospital under the diagnosis of acute epididymitis. IVP showed left hydronephrosis and round filling defect in the bladder. Ultrasonography and CT scan demonstrated a retrovesical cystic lesion. Right vesiculography under general anesthesia revealed a midline cystic lesion. Fenestration between the prostatic urethra and cystic lesion was performed by TUR-velmontanum. However, that cystic lesion did not disappeared. Our case seems to be the youngest case of urogenital sinus cyst reported in the Japanese literature.