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 (LGF), a light radiator (VHOE), a first light source (LS1) and a light collimator (WLC). The light guide film (LGF) includes a light entering part (LIN) defined at one side, a light guiding part (LWG) extending from the one side to an opposite side of the one side, and a light radiating part (LOT) defined on one plane surface. The light radiator (VHOE) is disposed on the light radiating part (LOT). The first light source (LS1) is disposed as facing the light entering part (LIN). The light collimator (WLC) is disposed on the light entering part (LIN) as facing the first light source (LS1). The light collimator (WLC) receives an expanding light from the first light source (LS1), converts the expanding light into a collimated light, and provides the collimated light to the light entering part (LIN).