A 95-year-old woman with no prior cardiac history presented with symptomatic complete atrioventricular block. She underwent temporary cardiac pacing via the cervical vein, but a pacing lead could not be introduced via the usual route because of a mediastinal tumor. A leadless pacemaker (MicraTM; Medtronic, Minneapolis, MN) was implanted on the right ventricular septum via the right femoral vein. The procedure time was 40 minutes without any complication. The implantation of a leadless pacemaker was useful to improve the symptoms of a super-elderly woman with mediastinal tumor.