A back light structure for a keyboard with back light includes a supporting plate, a light guiding plate, a flexible circuit board, and a light-emitting component. Keyswitch structures of the keyboard are disposed on the supporting plate. The light guiding plate is disposed under the supporting plate and has a through hole and a lateral surface. The circuit board adheres onto both a lower surface of the light guiding plate and a bottom surface of the supporting plate and covers the lateral surface. The light-emitting component is disposed on the circuit board in the through hole. Thereby, the circuit board also performs as a light shading structure for the lateral surface. A light reflection sheet need not cover both the lateral surface and the circuit board for avoiding light leakage but adheres onto only the lower surface, conducive to reducing the structural thickness of the back light structure.
