PURPOSE: To detect accurately the number of mobile tracks even in case an optical spot passes over the retrieving signal, by forming a retrieving signal part with a part having the groove width (a) equal to a guide groove part and a part having the groove width (b) and satisfying the specific relation to a track pitch P.
CONSTITUTION: For a light spot guide layer 43 consisting of a guide groove part 45 having recesses and projections and a retrieving signal part 48, a track 50 through which the light spot follows is formed with the projection parts when viewed from a lens 47 which condenses the light spot. The part 48 is used as a part of the track 50 and at the same time the part 48 includes a part 48a having the groove width (a) and a part 48b of the groove width (b) against the groove width (a) of the part 45. Thus the relation 'a - P/2 = P/2 - b' is satisfied to a track pitch P. As a result, the track control signal 53 is not affected even though a light spot passes over the part 48. Thus the accurate number of mobile tracks can be detected in case the information is retrieved at a high speed by means of a groove counting method.