A backlight unit (2) comprising a shallow box type case (3) having low profile sidewalls and opening upward, a light guide plate (7) in the box type case (3), and a supporting frame (11) containing optical members such as a linear light source (8) and an optical sheet (10) and securing the optical members in place by being fitted in the opening. A first engaging portion (4) is provided on the inside of the sidewall of the box type case (3) and second engaging portions (12.sub.1-12.sub.4) are provided on the outer surface of the sidewall of the supporting frame (11) being fitted in the upper opening of the box type case (3). A locking means (14) is attached to the second engaging portion such that the locking means (14) engages with and locks the first engaging portion when the first and second engaging portions engage with each other, thus preventing unfastening of the supporting frame. A liquid crystal display employing the backlight unit is also provided.