To provide a light source device which achieves substantial cooling effect to a mouthpiece member of a discharge lamp.

A light source device includes: a glass tube (25) housing an electrode for electric discharge; a discharge lamp having a mouthpiece part (28) connected with the glass tube (25); and a connection cable (72). A ring shaped member (74A) of the connection cable (72) is attached to the mouthpiece part (28) of the discharge lamp, and a ventilation path (74Ac) for cooling air is formed at the ring shaped member (74A). Electric power is supplied from a power source (32) to the mouthpiece part (28) through an electric cable (33D) of the connection cable (72) and the ring shaped member (74A). Cool air is supplied from a blower (34) to the mouthpiece part (28) through a duct for ventilation (35D) of the connection cable (72) and the ring shaped member (74A).