An electroimpedance tomograph including a plurality of electrodes placed on a body, measuring circuits processing voltage signals recorded by the electrodes, and a control unit supplying two electrodes each with an alternating current. The control unit has a preset feed frequency. The control unit processes the processed voltage signals of all other electrodes to reconstruct the impedance distribution of the body in the electrode plane. The control unit provides for an automatic setting of the current feed frequency/frequencies to record a background frequency spectrum by detecting the electrode voltage signals over a preset period of time and record them as a voltage time series and transform them into a frequency spectrum. The control unit searches in the background frequency spectrum for a frequency (different states) or frequencies (different frequencies) that leads to a useful signal to background ratio above a preset threshold value when used as a feed frequency.