A light irradiator includes a light source and a light guide to guide a light flux from the light source to an illumination area. The light guide has an incident surface, an emission surface and two side surfaces. The emission surface is located apart from a reading optical axis with an inclination relative to the reading optical axis. The following relations are satisfied: S1(x)≧S2(x) S1(L)>S2(L) wherein S1 represents a divided cross-sectional area near the reading optical axis and S2 represents a divided cross-sectional area far from the reading optical axis when a virtual cross-sectional area S parallel to the incident surface of the light guide is divided by a plane passing the center of the incident surface, formed by a direction perpendicular thereto and the incident surface and the main scanning direction; S (x) represents a virtual cross-sectional area distant from the incident surface of the light guide by x; L is a distance from the incident surface to the emission surface; and 0≦x