There is provided a slim liquid crystal display unit having a large screen by providing a slim structure capable of sufficiently radiating the heat from a light source. The liquid crystal display unit according to the present invention includes: a liquid crystal panel; an optical guiding board installed on a rear face thereof; a pair of light source-mounted substrate disposed, opposing right and left side faces thereof; a light source mounted on the light source-mounted substrate; a metal frame connected on the opposite side to the optical guiding board of the light source-mounted substrate; a chassis member for supporting the optical guiding board and a reflection sheet group; and a thermal diffusion member fixed onto the metal frame along with the chassis member. The chassis member has a stepped portion in the vicinity of the light source. Between the stepped portion and the reflection sheet group, a thermal insulation member is attached.