To suppress generation of a speckle pattern caused when projecting an image using light output from a light source device.

Radiated light 300 radiated from a semiconductor laser device 20b enters into and penetrates through a triangular prism 240 and enters into a half mirror 250. A part of the radiated light 300 which entered into the half mirror 250 is reflected and the residue penetrates through the half mirror 250 and enters into a light guide part 150. The penetrating light 302 is reflected by a reflection surfaces 410, 420 and 430 to circulate in the light guide part 150 and enters into the half mirror 250 again. A part of the penetrating light 302 which entered into the half mirror 250 penetrates through the half mirror 250, and the residue circulates in the light guide part 150 and enters into the half mirror 250 again. Thus, the light having the optical path difference longer than the coherent length and various optical path difference differing according to the number of times of circulation in the light guide part, thereby suppressing generation of a speckle pattern.