The invention relates to a light source unit control method and a light source control unit, and a light source unit comprising a filament lamp with a characteristic of having a light quantity changing almost in proportion to a control voltage, which comprise: a current detection sensor connected in series to the lamp; a multiplication circuit for generating a voltage V.sub.w proportionate to the product of a terminal voltage V of the lamp and an output voltage V.sub.1 of said current detection sensor; a multiplication circuit for generating a voltage V.sub.WW proportionate to the square root of said voltage V.sub.W; an error amplifier circuit for controlling an error to the minimum by comparing the output voltage V.sub.WW of said multiplication circuit and the control voltage V.sub.1 and a constant voltage power source unit for supplying an electric power to the filament lamp, the unit being connected to an output terminal of the error amplifier circuit, wherein the light quantity of the lamp is controlled by controlling the control voltage.