Abstract: An in-core nuclear instrumentation apparatus is provided with a neutron flux current detection circuit which converts a neutron flux current signal fed from a neutron detector into a voltage signal and amplifies the voltage signal with a capability to continuously vary amplifier gain. The neutron flux current detection circuit includes a current sensing resistor for converting the neutron flux current signal into the voltage signal, an amplifier for amplifying the voltage signal, wherein an equivalent resistance value of a feedback circuit of the amplifier can be controllably varied, an equivalent resistance control circuit for varying the equivalent resistance value of the feedback circuit, and an output circuit which outputs an output terminal voltage signal of the amplifier.