A backlight unit comprising: a lower frame having an approximately rectangular flat surface, a first pair of longitudinal opposite side surfaces, and a second pair of short opposite side surfaces, and defining an opening portion in its upper portion; a light source arranged in the opening portion; a light guide plate arranged in the opening portion for guiding light from the light source; and an upper frame having an approximately rectangular flat surface, a first pair of opposite longitudinal side surfaces, a second pair of opposite short side surfaces, and a window portion capable of transmitting a light from the light guide plate and arranged at an opening portion side of the opening portion, wherein the planar size of the upper frame is larger than the planar size of the lower frame; the lower frame and the upper frame being engagingly fixed at an engaging portion having i) a projecting portion integrally arranged on one of the first pair of side surfaces of the lower frame and ii) a rib portion extending from one of the first pair of side surfaces of the upper frame to the corresponding one of the first pair of side surface of the lower frame, the rib portion of the upper frame defining a hole portion with which the projecting portion of the lower frame can be engaged; wherein when taking a side view, a lower portion of the side surfaces of the upper frame and an upper portion of the side surfaces of the lower frame are in contact with each other and a width of the hole portion of the upper frame is larger than a width of the projecting portion of the lower frame, each of the projecting portion and the hole portion having opposite side end portions, when a side end portion of the projecting portion of the lower frame is made in contact with an opposing side end portion of the hole portion of the upper frame, one of the second pair of side surfaces of the upper frame is located approximately on a same plane as corresponding one of the second pair of side surfaces of the lower frame, and the other of the second pair of side surfaces of the upper frame is located outside of the corresponding other of the second pair of side surfaces of the lower frame, and said side end portion of the projecting portion and said side end portion of the hole portion that are being in contact with each other are closer than the other side end portion of the projecting portion and the other side end portion of the hole portion, to said one of the second pair of side surfaces of the upper frame and said corresponding one of the second pair of side surfaces of the lower frame that are located approximately on the same plane. a lower frame having an approximately rectangular flat surface, a first pair of longitudinal opposite side surfaces, and a second pair of short opposite side surfaces, and defining an opening portion in its upper portion; a light source arranged in the opening portion; a light guide plate arranged in the opening portion for guiding light from the light source; and an upper frame having an approximately rectangular flat surface, a first pair of opposite longitudinal side surfaces, a second pair of opposite short side surfaces, and a window portion capable of transmitting a light from the light guide plate and arranged at an opening portion side of the opening portion, wherein the planar size of the upper frame is larger than the planar size of the lower frame; the lower frame and the upper frame being engagingly fixed at an engaging portion having i) a projecting portion integrally arranged on one of the first pair of side surfaces of the lower frame and ii) a rib portion extending from one of the first pair of side surfaces of the upper frame to the corresponding one of the first pair of side surface of the lower frame, the rib portion of the upper frame defining a hole portion with which the projecting portion of the lower frame can be engaged; wherein when taking a side view, a lower portion of the side surfaces of the upper frame and an upper portion of the side surfaces of the lower frame are in contact with each other and a width of the hole portion of the upper frame is larger than a width of the projecting portion of the lower frame, each of the projecting portion and the hole portion having opposite side end portions, when a side end portion of the projecting portion of the lower frame is made in contact with an opposing side end portion of the hole portion of the upper frame, one of the second pair of side surfaces of the upper frame is located approximately on a same plane as corresponding one of the second pair of side surfaces of the lower frame, and the other of the second pair of side surfaces of the upper frame is located outside of the corresponding other of the second pair of side surfaces of the lower frame, and said side end portion of the projecting portion and said side end portion of the hole portion that are being in contact with each other are closer than the other side end portion of the projecting portion and the other side end portion of the hole portion, to said one of the second pair of side surfaces of the upper frame and said corresponding one of the second pair of side surfaces of the lower frame that are located approximately on the same plane.