To provide a compact light source device which emits a stable luminous flux.

The light source device includes: a light source 11 including a surface emitting laser array; a package member 21 that holds the light source 11; and a cap member 22 that is attached to the package member 21 to seal the light source 11. The cap member 22 has two windows (22a, 22b). The window 22a is provided in the +W direction of the light source 11 so as to be inclined 45 degrees with respect to an MZ plane and is covered by a half mirror 23. The window 22b is provided in the -Z direction of the half mirror 23 and is covered with a transparent member 24. Accordingly, while the light beam for writing-in outputted from the light source 11 and passing through the half mirror 23 passes through the window 22a, a light beam for monitoring reflected by the half mirror 23 passes through the window 22b.