A nanopore device includes an aperture and an electrode pair. A transimpedance amplifier converts a current signal IS that flows through the nanopore device into a voltage signal VS. The nanopore device measures small particles based on first data obtained by removing a DC component from the voltage signal VS and second data obtained based on the voltage signal VS from which the DC component has not been removed. Furthermore, the nanopore device is capable of monitoring the state of the nanopore device.