An information storage system for recording and reproducing information by use of irradiating light on a medium capable of overwriting information is provided. The information storage system is capable of recording information even if the light intensity is varied so as to enhance reliability. The intensity of the light at the time of overwriting is modulated, the more intense light intensity being PH and the less intense light intensity being PL. The system includes a mechanism to keep relationship of PH+ alpha PL equal to a constant ( alpha being a constant depending on the recording conditions). The laser drive circuit is so adjusted that PH and PL always provide an optimal condition even if PH and/or PL vary. Thus recording can always be performed under optimal conditions to enhance the reliability of the information storage system to expand the power margin of a recording medium and to increase the percentage yield of information medium.
