A fault detection apparatus for a rotor winding of a rotary machine comprises a peak value hold circuit, a peak value processor and a monitor. The peak value hold circuit holds peak value of a pulsing magnetic flux waveform supplied from a magnetic flux sensor mounted in proximity to a surface of a rotor, for a necessary time period. The peak value processor sequentially compares the peak value held in the peak value hold circuit with corresponding one of pre-stored peak values of the pulsing magnetic flux waveform to determine if a difference therebetween exceeds a preset level. The monitor indicates the presence or absence of fault based on the result of the comparison of the processor.