The present invention separates MTF into a plus frequency region and a minus frequency region and uses the plus frequency region and the minus frequency region to thereby enable an effectively usable frequency region to be doubled in effect. Digital information to be recorded on an optical disk medium is recorded as independent signals of, respectively, a light beam reflected or diffracted forward and a light beam reflected or diffracted backward with respect to a direction of traveling of a track on the optical disk medium. At the time of producing an optical disk medium, digital information is divided into two pieces, a complex modulating signal is generated by changing a plus frequency component and a minus frequency component according to the pieces of the divided digital information, and a laser beam is modulated on the basis of the complex modulating signal. At the time of reproducing the optical disk medium, part of the digital information is decoded from a light beam reflected or diffracted forward with respect to a direction of traveling of a track, and another part of the digital information is decoded from a light beam reflected or diffracted backward.