When a ratio R between a total angle of a widening angle in a median intensity of light of a light source (a GaN based semiconductor laser) and a total angle 2/ of a widening angle of light defining a numerical aperture NA of a collimator optical system (collimator lens) is defined as R=(sin^-1NA)2/, the numerical aperture of the collimator optical system (collimator lens) is set so that 2.0R0.58. Thus, an image exposure device is provided that can suppress stray light of a light source that emits a large amount of stray light.