A light source device to illuminate a target to be imaged by an imaging unit which includes N kinds of light detection elements which wavelength sensitivity regions are different each other includes a light source section, an insertion portion, an illumination light emitting portion and a light guide member. The light source section emits M kinds of narrow-band light which have different peak wavelengths, a difference between the peak wavelengths being equal to or more than an effective wavelength gap. The insertion portion is configured to be inserted into an internal space of an object. The illumination light emitting portion is provided in the insertion portion and which emits an illumination light. The light guide member guides the narrow-band light emitted from the light source section to the illumination light emitting portion.