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 (71, 73) and second-stage planetary gears (75, 76) such that the first-stage planetary gears (71, 73) are disposed at an upper portion and rotated in mesh with a second gear of the final gear and the second-stage planetary gears (75, 76) 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 (71, 73) 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 (71, 73) and second-stage planetary gears (75, 76) such that the first-stage planetary gears (71, 73) are disposed at an upper portion and rotated in mesh with a second gear of the final gear and the second-stage planetary gears (75, 76) 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 (71, 73) 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 (71, 73) of the planetary gear set; and a first sun gear protruding from a position corresponding to a center of the first rotational space.