An evaluation pattern, for evaluating the extent of occurrence of thermal interference, is recorded with recording power being varied stepwise from a low recording power to a high recording power. This test-written evaluation pattern is played back, and levels of occurrence of thermal interference are quantified for each recording power level of the recording. Then, recording power is set to a value such that a quantified value therefor is smaller than a pre-specified slice level. For the slice level, a measured value of PI errors (correctable errors) for carrying out acceptable recording is found, and the slice level is determined by finding a variation amount that corresponds to this measured value of PI errors.