An automatic transmission with an engine rpm sensor fault detector includes a starter switch (18); a clutch (6) for transmitting an output of an engine (8); a clutch control unit for controlling engagement of the clutch; a transmission (1); an engine rpm sensor (9); a an input shaft sensor (5a); a gear position sensor (40a) for detecting a gear position of the transmission; a control unit (4) for controlling the clutch control unit based on outputs of the respective detectors and enables the starter switch to operate when the transmission is in a neutral position. The control unit includes a starter switch detector (40b) for determining whether the starter switch is on and a fault determination unit (40c) for determining whether the first detector is faulty. The fault determination unit controls the clutch control unit to engage the clutch temporarily if there is no output from the first detector with the starter switch on and determines that the first detector is faulty if there is an output from the second detector.