A bioelectrical impedance apparatus and method includes a pair of current-carrying electrodes and a pair of measuring electrodes are rectilinearly arranged on a living body surface so that the measuring electrode having a smaller area than the other measuring electrode is placed very close to the current-carrying electrode having a smaller area than the other current-carrying electrode. The potential of the measuring electrode is substantially equal to the potential at a contact position of the current-carrying electrode, and the potential of the measuring electrode is substantially equal to the potential at a deepest position of a vertical part inside the living body. When a constant current flows between the current-carrying electrodes, a voltage between the measuring electrodes is measured by a detector so that a bioelectrical impedance of the vertical part may be calculated from a current, and a potential difference between upper and lower ends of the vertical part.