Rectification function has been observed in a CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ), which shows the giant tunneling magneto-resistance effect. The junction size was about 100nm x 200nm. Applied radio frequency (RF) current exerts a spin-torque interaction on magnetization in the MTJ, and causes resonant precession of spins. The MTJ shows high resistance only for one direction of the applied RF current and yields dc voltage as a result. The output voltage would be larger than those of semiconductor diodes if critical voltage to switch magnetization is smaller than 25mV. We named this device a "spin-torque diode". New applicatons of spin-tronics devices such as high frequency devices, are expected.