A flat light guide (1) for a vehicle lighting device, which includes a rear end (10) and a front end with a light output surface (13), the light guide (1) being defined between the two ends by a pair of side surfaces (11, 12). The light guide (1) has a base plane (X-Y) and at its rear end (10) is provided with at least one collimating element (2) arranged transversely or obliquely with respect to the plane (X-Y). Opposite the collimating element (2) is arranged an obliquely inclined reflective surface (3) to reflect light from the collimating element (2) into the flat light guide (1). The collimating element (2) is adapted for the light input from the light source into the light guide (1) which further guides the light between the side surfaces (11, 12) to the output surface (13).