An optical memory medium (2) comprises cores (21) constituting flat optical waveguides and clads (22) holding the cores between them, and has, at the interface of a core (21) and a clad (22) or in a core (21), a data image (203) in which data is recorded as a scattering factor and a pair of positioning marks (201, 202) that are scattering factors respectively needed for positioning. A reading beam (103) advances through a core (21) in an expanding manner, is scattered by and interfered with a data image (203), and data is reproduced from a data reproduction beam (1031) produced by it. A pair of positioning beams (101, 102) are shone into a core (21) with an offset in opposite directions with respect to the reading beam (103) and in the thickness direction of a core (21), and respectively scattered and interfered at a pair of positioning marks (201, 202). The incident position at a core (21) in its thickness direction of a beam emitted from a light source (11) is controlled based on the respective intensities of a pair of positioning mark beams (1011, 1021) thus produced.