Disclosed are a backlight unit for planar lighting apparatuses and a manufacturing method thereof. The backlight unit simplifies a layer structure to be formed into the shape of a vehicle interior trim part, and has a thin thickness and improved flexibility. The backlight unit includes a light emitting device configured to emit light; a light guide panel having polycarbonate and configured to disperse the light received from a light emitting device; a reflective sheet having fibers and provided below the light guide panel to reflect the light dispersed by the light guide panel; and an acrylic adhesive sheet interposed between the light guide panel and the reflective sheet. The reflective sheet includes pores.