An actuator having a double-gear structure for an electromechanical parking brake, the actuator comprising: a main housing having a motor seat and a gear seat ; a motor assembly disposed in the motor seat and providing torque; and a gear assembly rotated by the torque from the motor assembly and performing a reduction function through a plurality of gears, wherein the gear assembly includes; an idle gear connected to the motor assembly; a final gear separably disposed in the gear seat , and having a first gear formed on an outer circumferential surface and rotating in mesh with the idle gear ; a ring gear disposed in the gear seat under the final gear , separably fixed to an installation frame disposed in the main housing , and having revolution gear teeth on an inner side; and a planetary gear set composed of first-stage planetary gears and second-stage planetary gears such that the first-stage planetary gears are disposed at an upper portion and rotated in mesh with a second gear of the final gear and the second-stage planetary gears are disposed at a lower portion and rotated in mesh with the revolution gear teeth of the ring gear , characterized in that the final gear includes: a gear body; the first gear formed on an outer circumferential surface of the gear body and engaging with the idle gear ; the second gear formed on an inner circumferential surface of a first rotational space recessed on a bottom of the gear body and engaging with the first-stage planetary gears of the planetary gear set ; and a first sun gear protruding from a position corresponding to a center of the first rotational space . a main housing having a motor seat and a gear seat ; a motor assembly disposed in the motor seat and providing torque; and a gear assembly rotated by the torque from the motor assembly and performing a reduction function through a plurality of gears, wherein the gear assembly includes; an idle gear connected to the motor assembly; a final gear separably disposed in the gear seat , and having a first gear formed on an outer circumferential surface and rotating in mesh with the idle gear ; a ring gear disposed in the gear seat under the final gear , separably fixed to an installation frame disposed in the main housing , and having revolution gear teeth on an inner side; and a planetary gear set composed of first-stage planetary gears and second-stage planetary gears such that the first-stage planetary gears are disposed at an upper portion and rotated in mesh with a second gear of the final gear and the second-stage planetary gears are disposed at a lower portion and rotated in mesh with the revolution gear teeth of the ring gear , characterized in that the final gear includes: a gear body; the first gear formed on an outer circumferential surface of the gear body and engaging with the idle gear ; the second gear formed on an inner circumferential surface of a first rotational space recessed on a bottom of the gear body and engaging with the first-stage planetary gears of the planetary gear set ; and a first sun gear protruding from a position corresponding to a center of the first rotational space . a gear body; the first gear formed on an outer circumferential surface of the gear body and engaging with the idle gear ; the second gear formed on an inner circumferential surface of a first rotational space recessed on a bottom of the gear body and engaging with the first-stage planetary gears of the planetary gear set ; and a first sun gear protruding from a position corresponding to a center of the first rotational space . <EOS>