an optical reproducing method for reproducing information on an optical medium , including a disk - like substrate , a spiral or concentric - shaped groove structure along tracks formed on the disk - like substrate with each of the tracks being divided into a plurality of units including a dropout portion in a circumferential direction of the groove structure . the dropout portion is a non - groove portion . the groove structure is formed with a wobble in a fixed cycled in a radial direction with the fixed cycle of the wobble continuing in the circumferential direction along the track . each length of the units is an integer multiple of the cycle of the wobble . the method includes irradiating an optical spot on the optical medium , detecting a reflected beam from the optical medium , and reproducing information on the optical medium by using a signal obtained by the reflected beam .