We report an unusual case of a 14-year old boy who presented with proteinuria and pyuria detected in a medical checkup at school. After denial of kidney disease, computed tomography of the pelvis showed a bladder stone with an internal low density and urethroscopy showed an odd stick at the prostatic urethra. Because of the failure of removal by the transurethral technique, he underwent suprapubic cystostomy against the foreign body stuck into the prostatic urethra. After surgery, he admitted that he had self-introduced a sewing instrument into the bladder for the purpose of masturbation one year three months previously.