An apparatus for generating an electron beam is disclosed to reduce emittance of an electron beam. The apparatus includes: a housing including a rear portion where an electron beam is generated, a front portion having an electron beam discharge hole for discharging the electron beam to the exterior, and a side portion connecting the rear portion and the front portion, the side portion having a first hole and an opposite side portion, facing the first hole, having a second hole in order to reduce asymmetry of an electric field caused by the first hole; and a waveguide installed on the side portion to supply an electromagnetic wave to the interior of the housing through the first hole, wherein the electron beam is generated by laser incident to the interior of the housing and accelerated by the electromagnetic wave supplied to the interior of the housing.