A light-emitting unit comprising: a plurality of light-emitting sections; a setter that is electrically connected to the plurality of light-emitting sections and that sets, among the plurality of light-emitting sections, a light-emitting section to a state in which the light-emitting section is able to emit light; a first terminal that is adapted to receive a first voltage to be supplied to the set light-emitting section; and a second terminal that is adapted to receive a second voltage to be supplied to the setter, the second voltage being lower than the first voltage, the plurality of light-emitting sections are disposed on a portion of a substrate, the portion including a center of the substrate, the setter is disposed at a peripheral portion of the substrate, the light-emitting unit being characterized in that: each of the plurality of light-emitting sections includes a light-emitting diode and a drive thyristor, the drive thyristor being connected in series with the light-emitting diode and driving the light-emitting diode, an anode of the light-emitting diode and a cathode of the drive thyristor are connected to each other, the drive thyristor and the light-emitting diode are stacked on each other, the setter includes a plurality of setting elements and shifts a setting element which is to be turned ON between the plurality of setting elements so as to set a corresponding drive thyristor to be an ON state, the corresponding drive thyristor being a drive thyristor included in a light-emitting section which is to emit light among the plurality of light-emitting sections, and the setter includes: a first part that receives the first voltage, a second part that receives a reference potential, the reference potential being different from the first voltage and the second voltage, and an insulator interposed between the first part and the second part. a plurality of light-emitting sections; a setter that is electrically connected to the plurality of light-emitting sections and that sets, among the plurality of light-emitting sections, a light-emitting section to a state in which the light-emitting section is able to emit light; a first terminal that is adapted to receive a first voltage to be supplied to the set light-emitting section; and a second terminal that is adapted to receive a second voltage to be supplied to the setter, the second voltage being lower than the first voltage, the plurality of light-emitting sections are disposed on a portion of a substrate, the portion including a center of the substrate, the setter is disposed at a peripheral portion of the substrate, the light-emitting unit being characterized in that: each of the plurality of light-emitting sections includes a light-emitting diode and a drive thyristor, the drive thyristor being connected in series with the light-emitting diode and driving the light-emitting diode, an anode of the light-emitting diode and a cathode of the drive thyristor are connected to each other, the drive thyristor and the light-emitting diode are stacked on each other, the setter includes a plurality of setting elements and shifts a setting element which is to be turned ON between the plurality of setting elements so as to set a corresponding drive thyristor to be an ON state, the corresponding drive thyristor being a drive thyristor included in a light-emitting section which is to emit light among the plurality of light-emitting sections, and the setter includes: a first part that receives the first voltage, a second part that receives a reference potential, the reference potential being different from the first voltage and the second voltage, and an insulator interposed between the first part and the second part. each of the plurality of light-emitting sections includes a light-emitting diode and a drive thyristor, the drive thyristor being connected in series with the light-emitting diode and driving the light-emitting diode, an anode of the light-emitting diode and a cathode of the drive thyristor are connected to each other, the drive thyristor and the light-emitting diode are stacked on each other, the setter includes a plurality of setting elements and shifts a setting element which is to be turned ON between the plurality of setting elements so as to set a corresponding drive thyristor to be an ON state, the corresponding drive thyristor being a drive thyristor included in a light-emitting section which is to emit light among the plurality of light-emitting sections, and the setter includes: a first part that receives the first voltage, a second part that receives a reference potential, the reference potential being different from the first voltage and the second voltage, and an insulator interposed between the first part and the second part. a first part that receives the first voltage, a second part that receives a reference potential, the reference potential being different from the first voltage and the second voltage, and an insulator interposed between the first part and the second part.