A brake apparatus for a vehicle, comprising: a caliper body to be fixed to a vehicle body; a driving unit fixed to the caliper body, and generates a rotational force; a piston unit movably installed in the caliper body, and presses or releases a brake pad according to a moving direction of the piston unit; a transfer gear unit connected to the driving unit, and rotated by the rotational force received from the driving unit; and a planetary gear unit fixed to the caliper body, and moves the piston unit forward and backward in connection with a rotation of the transfer gear unit; wherein the planetary gear unit comprises: a ring gear; a sun gear rotatably supported by the ring gear, and engaged with and coupled to the transfer gear unit; a plurality of planetary gears engaged with and coupled to the ring gear and the sun gear, and rotates and revolves in connection with a rotation of the sun gear; and a carrier connected to the planetary gears, and transfers rotational forces of the planetary gears to the piston unit; wherein the ring gear comprises: a fixed part fixed to the caliper body; and a reducer disposed coaxially with the fixed part, such that an inner circumferential surface of the reducer is engaged with and coupled to the planetary gears; characterized in that a support part of the ring gear is disposed between the fixed part and the reducer, and extended inward in a radial direction of the ring gear. a caliper body to be fixed to a vehicle body; a driving unit fixed to the caliper body, and generates a rotational force; a piston unit movably installed in the caliper body, and presses or releases a brake pad according to a moving direction of the piston unit; a transfer gear unit connected to the driving unit, and rotated by the rotational force received from the driving unit; and a planetary gear unit fixed to the caliper body, and moves the piston unit forward and backward in connection with a rotation of the transfer gear unit; wherein the planetary gear unit comprises: a ring gear; a sun gear rotatably supported by the ring gear, and engaged with and coupled to the transfer gear unit; a plurality of planetary gears engaged with and coupled to the ring gear and the sun gear, and rotates and revolves in connection with a rotation of the sun gear; and a carrier connected to the planetary gears, and transfers rotational forces of the planetary gears to the piston unit; wherein the ring gear comprises: a fixed part fixed to the caliper body; and a reducer disposed coaxially with the fixed part, such that an inner circumferential surface of the reducer is engaged with and coupled to the planetary gears; characterized in that a support part of the ring gear is disposed between the fixed part and the reducer, and extended inward in a radial direction of the ring gear. a ring gear; a sun gear rotatably supported by the ring gear, and engaged with and coupled to the transfer gear unit; a plurality of planetary gears engaged with and coupled to the ring gear and the sun gear, and rotates and revolves in connection with a rotation of the sun gear; and a carrier connected to the planetary gears, and transfers rotational forces of the planetary gears to the piston unit; wherein the ring gear comprises: a fixed part fixed to the caliper body; and a reducer disposed coaxially with the fixed part, such that an inner circumferential surface of the reducer is engaged with and coupled to the planetary gears; characterized in that a support part of the ring gear is disposed between the fixed part and the reducer, and extended inward in a radial direction of the ring gear. a fixed part fixed to the caliper body; and a reducer disposed coaxially with the fixed part, such that an inner circumferential surface of the reducer is engaged with and coupled to the planetary gears; characterized in that a support part of the ring gear is disposed between the fixed part and the reducer, and extended inward in a radial direction of the ring gear.