A light source device that includes a first resistor that is connected to a given potential, a light emitting element that is connected in series to the first resistor, a second resistor that is connected to the given potential, and a first current source that is connected in series to the second resistor and that is configured to supply a freely-selected current within a given range are included. A first voltage is taken out from a first connection part where the first resistor and the light emitting element are connected to each other and a second voltage is taken out from a second connection part where the second resistor and the first current source are connected to each other.