In one embodiment, a beam detector includes a first aperture plate including a first passage hole, a second aperture plate including a second passage hole that allows a single detection target beam passing through the first passage hole to pass therethrough, and a sensor detecting a beam current of the detection target beam passing through the second passage hole. The second aperture plate includes an electrically conductive material, a plurality of third passage holes are formed around the second passage hole, and the plurality of third passage holes allow light to pass therethrough.