A backlight device, comprising: a light guide plate; and a plurality of light source units, placed side by side at placement intervals of P2 in a direction extending along an end face of the light guide plate, wherein each of the plurality of light source units includes: a plurality of white light-emitting diodes placed side by side at placement intervals of P1 in the direction extending therealong, the plurality of white light-emitting diodes being spaced apart from the end face of the light guide by a distance H; and a light mixing chamber is formed between the light-emitting diodes and the end face of the light guide plate, the light mixing chamber causing light emitted from the light-emitting diodes to enter the end face of the light guide plate; a package section, on which the plurality of white light -emitting diodes are fixed, having a pair of inclined portions provided at both ends in the direction extending therealong, wherein the pair of inclined portions reflect white light radiated from the plurality of white light-emitting diodes so as to be guided to the end face thereof, and the height of the inclined portion is greater than that of the light-emitting diode, and wherein the light guide plate is so arranged as to meet a condition that P1≤H≤P2. a light guide plate; and a plurality of light source units, placed side by side at placement intervals of P2 in a direction extending along an end face of the light guide plate, wherein each of the plurality of light source units includes: a plurality of white light-emitting diodes placed side by side at placement intervals of P1 in the direction extending therealong, the plurality of white light-emitting diodes being spaced apart from the end face of the light guide by a distance H; and a light mixing chamber is formed between the light-emitting diodes and the end face of the light guide plate, the light mixing chamber causing light emitted from the light-emitting diodes to enter the end face of the light guide plate; a package section, on which the plurality of white light -emitting diodes are fixed, having a pair of inclined portions provided at both ends in the direction extending therealong, wherein the pair of inclined portions reflect white light radiated from the plurality of white light-emitting diodes so as to be guided to the end face thereof, and the height of the inclined portion is greater than that of the light-emitting diode, and wherein the light guide plate is so arranged as to meet a condition that P1≤H≤P2. a plurality of white light-emitting diodes placed side by side at placement intervals of P1 in the direction extending therealong, the plurality of white light-emitting diodes being spaced apart from the end face of the light guide by a distance H; and a light mixing chamber is formed between the light-emitting diodes and the end face of the light guide plate, the light mixing chamber causing light emitted from the light-emitting diodes to enter the end face of the light guide plate; a package section, on which the plurality of white light -emitting diodes are fixed, having a pair of inclined portions provided at both ends in the direction extending therealong, wherein the pair of inclined portions reflect white light radiated from the plurality of white light-emitting diodes so as to be guided to the end face thereof, and the height of the inclined portion is greater than that of the light-emitting diode, and wherein the light guide plate is so arranged as to meet a condition that P1≤H≤P2.