An operational amplifier and a scanning electron microscope which are capable of dealing with high voltage and large current, and which allow implementation of stable and precise amplification, the operational amplifier having a first-stage amplification unit including a differential pair, a base-grounded amplification circuit, and an active load, the base-grounded amplification circuit being cascode-connected to the differential pair a second-stage amplification unit including an inverter having an emitter follower circuit and a constant-current load circuit, and a third-stage amplification unit including a source follower circuit or an emitter follower circuit.