The stress-induced breakdown characteristics in magnesium oxide (MgO)-based magnetic tunnel junctions (MTJs) including an inserted Mg layer 0.5 nm thick is investigated, above or below an MgO tunnel barrier. Regardless of the insertion position, the inserted layer suppressed the time-dependent dielectric breakdown (TDDB) observed in the case of electron tunnelling into the Mg-inserted interface. This indicates that the Mg insertion significantly suppressed trap site formation at the anode-side barrier/electrode interface. The improvement of TDDB by the Mg insertion was confirmed through constant voltage stress experiments. Therefore, interface modification by means of inserting an ultra-thin metallic layer is highly effective in improving the reliability of an MTJ tunnel barrier for practical applications.