A generation mechanism of high-energy electrons in the plasma sheet--lobe boundary region associated with magnetic reconnection is studied by using 2-1/2-dimensional kinetic simulations in a large system. Our simulation code employs the adaptive mesh refinement technique and the particle splitting algorithm to the conventional particle-in-cell code, which enable us to perform large-scale kinetic simulations. It is shown that the electron two-stream instability arising between the cold background electrons and the intense beam electrons with high temperature is responsible for the formation of the flat-topped electrons, which have been often observed in the boundary region when magnetic reconnection has occurred. Electrons are quickly heated along the ambient magnetic field due to the trapping effect in the electrostatic potential wells, and the shoulder energy of the flat-topped electron distribution reaches a few keV, consistent with satellite observations. The intense beam consists of high- and low-energy electrons in the perpendicular direction. The former electrons come from the lobe region through the vicinity of the X line, while the latter originate from the opposite boundary region of the plasma sheet. The electron two-stream instability in association with magnetic reconnection is also responsible for the generation of the electrostatic solitary waves that have been frequently detected in the boundary region. This indicates that the electrostatic turbulence should play an important role in strong electron energization in the plasma sheet--lobe boundary region during magnetic reconnection.