A diagnosis apparatus for a field winding type rotating electric machine having a field winding, an armature winding, and a direct current power supply that serves as a power supply source for the field winding and the armature winding, the diagnosis apparatus being applied to a rotating electric machine that operates as a motor or a power generator, and comprising: a diagnosis circuit (11, 12) that includes an internal power supply circuit having an open fault determination potential set at a different value to a positive electrode potential and a negative electrode potential of the direct current power supply so that in a fault diagnosis condition in which an induced current generated by a relative rotary motion of the armature winding does not flow through the field winding, the diagnosis circuit (11, 12) performs a fault diagnosis on a circuit including the field winding by measuring a voltage value of the circuit including the field winding that is generated when a current is passed through the field winding using the internal power supply circuit, wherein, on the basis of a measurement result of the voltage value of the circuit including the field winding in the fault diagnosis condition, the diagnosis circuit (11, 12): determines that a disconnection has occurred in the circuit when the measurement result is within an open fault determination voltage range including the open fault determination potential; determines that a power fault has occurred in the circuit when the measurement result is within a power fault determination voltage range including the positive electrode potential; and determines that an earth fault has occurred in the circuit when the measurement result is within an earth fault determination voltage range including the negative electrode potential. a diagnosis circuit (11, 12) that includes an internal power supply circuit having an open fault determination potential set at a different value to a positive electrode potential and a negative electrode potential of the direct current power supply so that in a fault diagnosis condition in which an induced current generated by a relative rotary motion of the armature winding does not flow through the field winding, the diagnosis circuit (11, 12) performs a fault diagnosis on a circuit including the field winding by measuring a voltage value of the circuit including the field winding that is generated when a current is passed through the field winding using the internal power supply circuit, wherein, on the basis of a measurement result of the voltage value of the circuit including the field winding in the fault diagnosis condition, the diagnosis circuit (11, 12): determines that a disconnection has occurred in the circuit when the measurement result is within an open fault determination voltage range including the open fault determination potential; determines that a power fault has occurred in the circuit when the measurement result is within a power fault determination voltage range including the positive electrode potential; and determines that an earth fault has occurred in the circuit when the measurement result is within an earth fault determination voltage range including the negative electrode potential. determines that a disconnection has occurred in the circuit when the measurement result is within an open fault determination voltage range including the open fault determination potential; determines that a power fault has occurred in the circuit when the measurement result is within a power fault determination voltage range including the positive electrode potential; and determines that an earth fault has occurred in the circuit when the measurement result is within an earth fault determination voltage range including the negative electrode potential.