PURPOSE: To provide a protective device preventing unnecessary operation of a ground relay and capable of safely maintaining and inspecting a field circuit and being used for the generator field circuit.

CONSTITUTION: A protective device for a generator field circuit has a ground relay 7 detecting the ground failure of the field circuit supplying the field winding 3 of a generator 1 with DC currents and exciting the generator 1, an output section 9 for opening the field breaker 4 of the field circuit on the basis of the actuating signal of the ground relay 7, and an unnecessary operation preventive circuit 8 being mounted between the ground relay 7 and the output section and preventing the output of the actuating signal of the ground relay 7 by the abnormal voltage of the field winding 3 generated in the stop process of the generator 1.