It is shown that the Pb(root 3x root 3)R30/Si(111) ordered layer is an excellent mirror for neutral He atoms. It focuses more than 15% of the incoming He atoms into the specular peak, and is stable up to 450 K. Moreover, the reflectivity remains almost unchanged in a time scale of several weeks in ultrahigh vacuum. As a consequence, this system is a very good candidate to be used as a mirror in the next generation of the scanning helium atom microscope.