A light irradiation device includes: a focus servo control unit including a light source, a spatial light modulating unit, and a light irradiating unit, for focus servo control so that the ideal focal position of light via an objective lens is such that distance between an ideal focal position and a hologram recording medium surface is smaller than distance from the surface to the lower layer side face of the recording layer, and the focal position of light via the objective lens is constant at the ideal focal position; an objective-lens/relay-lens distance adjusting unit to adjust distance between the objective lens and a relay lens close to the objective lens; a light reception unit to receive marker light; and a constant distance control unit to control the objective-lens/relay-lens distance adjusting unit based on error between the ideal position of the marker light, and the actual position of the marker light.