An illuminating device includes a light guide plate, a light source disposed on a side face of the light guide plate, and a light modulation element disposed on a surface or on inside of the light guide plate and joined to the light guide plate. The light modulation element has a pair of transparent substrates disposed apart from each other and opposed to each other, a pair of electrodes provided on surfaces of the pair of transparent substrates, and a light modulation layer provided in a gap between the pair of transparent substrates. The light modulation layer includes a first region and a second region both having optical anisotropy and having response speeds to an electric field different from each other.