Magnetic tunnel junctions (MTJs) with a stacking structure of ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}∕\mathrm{Al}–\mathrm{O}∕{\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ were fabricated using magnetron sputtering system. Fabricated MTJ exhibited an extremely large tunneling magnetoresistance (TMR) ratio of 570% at low temperature, which is the highest TMR ratio reported to date for an amorphous Al–O tunneling barrier. The observed dependence of tunneling conductance on bias voltage clearly reveals the half-metallic energy gap of ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ . The origins of large temperature dependence of TMR ratio were discussed on the basis of the present results.