An optical disk writing apparatus maintaining appropriate write power in real time with a simple configuration includes a buffer for recording data, a binarization circuit supplying data to the buffer, an LD writing data recorded in the buffer onto a DVD disk, a detector reading out from the DVD disk a signal representing data written by the LD at an arbitrary time during a period starting from when the amount of data recorded in the buffer and not yet written onto the DVD disk becomes zero until the amount of data reaches a predetermined number of ECC blocks, and a servo processor correcting an error in the data read out by the detector for every data amount susceptible to error correction at one time, evaluating the writing of the LD using the corrected signal, and controlling the energy density to be employed in writing data onto the DVD disk by the LD according to its own evaluation.