A failure detection apparatus for an alternator can detect a short circuit failure of a diode in a full-wave rectifier with high precision by use of a simple circuit structure based on the state of a detection terminal when the alternator generates no electricity. The rectifier circuit has a pair of diodes connected to a positive terminal and a negative terminal, respectively, of a battery. A failure detection circuit has the detection terminal connected to a P terminal or a neutral point of an armature coil, and makes a failure determination in the following manner. When in the non-power generation state of the armature coil, the detection terminal is in a floating or high impedance state, the full-wave rectifier circuit is determined to be normal, whereas when otherwise, the full-wave rectifier circuit is determined to be in a failure.