A light detector according to an embodiment includes a light receiver and a controller. The controller is configured to set first and second light-receiving regions. The first light-receiving region includes first and second pixel regions. The second light-receiving region includes a third pixel region. An area of the third pixel region is larger than a total area of the first and second pixel regions. The light receiver is configured to, when light is applied: cause each of the first and second pixel regions within the first light-receiving region to individually output a signal; and cause the third pixel region within the second light-receiving region to output signals collectively.