In order to set optimum laser power for a disk smoothly and appropriately with a small number of times of test writing, an integrated value Sn is obtained from each kind of laser power Pwn set for the test writing and the modulation factor m(Pwn) of a reflection light intensity obtained through the test writing with the laser power Pwn using an expression "Sn=m(Pwn).times.Pwn.sup.2", laser power Pwth is obtained at which the modulation factor m(Pwn) becomes zero when relational characteristics of the integrated value Sn and the laser power Pwn are linearly approximated, and optimum laser power Pw is set based on the laser power Pwth. Here, the optimum laser power Pw is obtained from the target .gamma. value of the disk using an expression "Pw=Pwth.times.{1+1/(.gamma.+1)}". The target .gamma. value is obtained from an ADIP of the disk.