PROBLEM TO BE SOLVED: To provide a light source device for an endoscope capable of obtaining an image of high precision without reducing the quantity of light of a specific wavelength region of white illumination light at the time of both of special light observation and usual observation while allowing to brighten the whole image, thereby enabling an advanced diagnosis.SOLUTION: The device includes: transmission unit for transmitting the white illumination light emitted from a first light source unit and a multiplexing member reflecting at least the narrow band light emitted from a second light source unit and provided at the central part of the emitting surface of the transmission unit, and whose length in the longitudinal direction is longer than that in the transverse direction crossing the longitudinal direction at a right angle while having a flat shape almost equal to (1/sin&thetas) times of the width of the luminous flux of the white illumination light. Thus, the white illumination light is transmitted to the transmission unit while reflecting the narrow band light with a reflection part, thereby multiplexing the white illumination light and the narrow band light.
