A gas concentration detector includes a gas sensor and an abnormality detecting section. The gas sensor includes a single solid electrolyte body having oxygen ion conductivity and electrodes formed on both surfaces of the single solid electrolyte body. The abnormality detecting section is configured to apply an AC voltage to a specific one of the electrodes and concurrently measure currents flowing respectively between the specific electrode and each of the other electrodes through the solid electrolyte body, and determine that at least one of wires respectively connected to the corresponding electrodes is broken if at least one of the measured currents is smaller than a predetermined value.