A backlight apparatus used in an illuminated keyboard includes a light-guiding plate, a circuit board and a plurality of light-emitting devices all electrically bonded on the circuit board. The light-guiding plate has a plurality of through holes which each corresponds to one of the light-emitting devices. The circuit board is disposed on a back face of the light-guiding plate such that each light-emitting device is disposed within the corresponding through hole, and that a light-emitting side face of each light-emitting device faces a light incident surface of the corresponding through hole. A specific through hole of the through holes has an optical adjustment surface on the light incident surface thereof, and the optical adjustment surface has a curved surface central axis coaxial with a central optical axis of the corresponding light-emitting device.