An attosecond electron beam generation is studied by an intense short-pulse TEM (1,0) + TEM (0,1)-mode laser with a plasma separator in vacuum. The TEM (1,0) + TEM (0,1)-mode laser has a ring-shaped intensity peak in the radial direction. Electrons are accelerated and compressed near the focus point of the TEM (1,0) + TEM (0,1)-mode laser. However, after the focus point, some electrons move to its deceleration phase of the laser pulse and are decelerated. As a result, a longitudinal velocity deference of electrons generated causes a density lowering. In order to suppress the deceleration and the density lowering, we set an overdense plasma-foil separator before the electrons move to the deceleration phase of the laser pulse. Since only the laser is reflected by the plasma separator, the electrons do not experience the deceleration phase and the density of the electron bunch is kept high after passing through the plasma separator. Consequently, a high-density electron beam is generated and at the same time, the pulse length of the electron bunch becomes sub-femto second.