In order to provide an optical device and a diffusion plate which has high transmittance and is capable of reducing speckle noise, the present it is a diffusion plate 1 in which a plurality of microlens cells 11 are arranged on both surfaces of a transparent substrate 10, the diffusion plate 1 having a first microlens array 12A which is formed on one surface of the transparent substrate 10 and comprises a plurality of concave or convex microlens cells 11, and a second microlens array 12B which is formed on the other surface on the reverse side from the one surface and comprises a plurality of concave or convex microlens cells 11, and the diffusion plate 1 being configured so that light emitted from the microlens cells 11 constituting the first microlens array 12A is incident on the microlens cells 11 constituting the second microlens array 12B.