To solve the problem that it is difficult to select a determination value for detecting abnormality since response characteristics of a detection amount change depending on an operation state of a synchronous generator body when a generator voltage and the other detection amount are compared in order to determine detection abnormality of the generator voltage.

An excitation controller of a synchronous generator comprises: voltage detection means 13 for obtaining a voltage detection value Vg from a generator voltage 12s of a synchronous generator 1; voltage change rate detection means 20 for detecting a change rate of the voltage detection value Vg; phase detection means 24 for detecting a voltage phase from the generator voltage 12s of the synchronous generator 1; means 25 for obtaining a change rate of the voltage phase and generating a comparison electricity quantity; and self-diagnostic means 23 for detecting that the generator voltage obtained from the synchronous generator is abnormal when the voltage change rate and the comparison electricity quantity deviate from a set value respectively.