Provision of a displacement sensor, which detects an amount of shift at an
objective lens, and a controller, which adjusts an emitted power based on
a value measured by the displacement sensor and compensation data stored
in a memory, permits detection by the displacement sensor of the amount
of shift at the objective lens as necessary during recording of data, and
the controller is capable of causing emission of an optimum emitted power
from a laser source to an optical disk based on the measured value
obtained and the compensation data. As a result, elimination of effects
of fluctuation in effective power due to objective lens shift is
permitted, and it is possible to ensure that the effective power of a
laser irradiating the active layer will always be maintained at an
optimum laser power.