Provided is a planar light source device (300) having a planar light source with uniform luminance distribution by using a laser light source for outputting a beam-like luminous flux. The planar light source device (300) applies a laser beam from the side surface of a light guide plate (310) through a scanning mirror (330) and periodically scans the entire light guide plate. At that time, a light output (P(t)) of the laser light source (320) is modulated in synchronization with the scanning cycle so that the output is proportional to the temporal differentiation dS/dt of a scanning area (S(t)) from a search start within a scanning period. Thus, changes of the scanning speed and scanning length with time are eliminated and planar lighting with uniform luminance can be performed.