The present disclosure relates to a viewing angle switchable back light unit in which the general mode and the privacy mode in a liquid crystal display can be selected. The present disclosure suggests a thin film type back light unit comprising: a light guide film including a light entering part disposed at one side, a light changing part disposed at other side and a light guiding part connecting the light entering part and the light changing part; a light radiating part disposed on an upper surface of the light guide film; and a light source disposed at the light entering part of the light guide film. The light entering part includes a wedge light coupler and a light deflection element. The light changing part includes a light reflecting pattern. The light radiating part includes a light radiating film providing some of the collimated lights as back lights.