PURPOSE: To constitute a beam flood device whose read depth is deep by providing a zone plate with a diffraction grating diffracting a divergent light beam from a light source.

CONSTITUTION: The zone plate 3 is a disk on which many concentric circular shaped diffraction gratings are formed, and intervals of these gratings become narrower with distance from a center of a concentric circle, and the central line of the concentric circle of the diffraction grating becomes an optical axis 4. When the divergent light beam with a fixed wavelength radiated from the light source 1 arranged on the optical axis 4 is made incident, the ring band plate 3 diffracts it, and converts a part of it to a +1st-order diffracted light beam and a +2nd-order diffracted light beam to emit. The +1st-order diffracted light beam and the +2nd-order diffracted light beam advances so as to be most converged on a converging point 11 and the converging point 21 on the optical axis 4 respectively. A range where a bar code with a prescribed bar width is readable is constituted of two of the range 12 and the range 22 around the converging point 11 and the converging point 21 respectively, and they are made adjacent by properly selecting the periodical distribution of the diffraction grating of the ring band plate 3.