To provide diagnostic light irradiation apparatuses for obtaining excellent light coupling efficiency between a light source device and a fiber probe regardless of the fiber probe to be connected.

The diagnostic light irradiation apparatuses (100, 200, 300) irradiate a prescribed part in a body with a diagnostic light for optically diagnosing the prescribed part of the body. These apparatus include: a light supply means (110) for supplying the diagnostic light; and a light transmitting means (104) which is attached/detached to/from the light supply means, and at least a part of which is inserted to the body in order to allow the prescribed part of the body to be irradiated with the diagnostic light supplied from the light supply means. The light transmitting means is formed to be able to be inserted into the body through a channel in the treatment implement of an endoscope, so that it is easily inserted into the body by using the existing endoscope. The apparatuses also include an adjusting means (132) for adjusting the optical coupling state of the light supply means with the light transmitting means arranged in the light supply means.