An eddy current speed reducer which includes: a brake disk that is fixed to a rotation shaft of a vehicle; permanent magnets that are disposed not only oppositely relative to a principal surface of the brake disk but also circumferentially in such a manner that each magnet is magnetized in reverse polarity to adjacent ones; a rotational member that retains the permanent magnets, has a switch brake disk, and is rotatably supported by the rotation shaft; a switch brake caliper that has brake pads sandwiching the switch brake disk therebetween, and is fixed to an irrotational portion of the vehicle; and an electric linear actuator that causes the brake pads to be linearly driven by converting rotational motion of an electric motor into linear motion, the speed reducer being characterized in that: the electric linear actuator is configured such that a plurality of planetary rollers are interposed between an outer circumferential surface of a rotor shaft, which rotates integrally with a main shaft of the electric motor, and an inner circumferential surface of an outer race member that concentrically surrounds the rotor shaft, and each of the planetary rollers revolves while rotating around the rotor shaft as a result of the rotation of the rotor shaft; while allowing a spiral convex zone to be disposed on the inner circumferential surface of the outer race member, provided on an outer circumferential surface of each of the planetary rollers is a circumferential groove system, which has the same pitch as the spiral convex zone and meshes with the spiral convex zone, or a spiral groove system, which has the same pitch as the spiral convex zone and a lead angle different from that of the spiral convex zone and meshes with the spiral convex zone; and the outer race member moves in an axial direction in response to revolving and orbital motion of each of the planetary rollers as a result of the rotation of the rotor shaft to thereby cause the brake pads to be linearly driven as a result of the axial movement of the outer race member. a brake disk that is fixed to a rotation shaft of a vehicle; permanent magnets that are disposed not only oppositely relative to a principal surface of the brake disk but also circumferentially in such a manner that each magnet is magnetized in reverse polarity to adjacent ones; a rotational member that retains the permanent magnets, has a switch brake disk, and is rotatably supported by the rotation shaft; a switch brake caliper that has brake pads sandwiching the switch brake disk therebetween, and is fixed to an irrotational portion of the vehicle; and an electric linear actuator that causes the brake pads to be linearly driven by converting rotational motion of an electric motor into linear motion, the speed reducer being characterized in that: the electric linear actuator is configured such that a plurality of planetary rollers are interposed between an outer circumferential surface of a rotor shaft, which rotates integrally with a main shaft of the electric motor, and an inner circumferential surface of an outer race member that concentrically surrounds the rotor shaft, and each of the planetary rollers revolves while rotating around the rotor shaft as a result of the rotation of the rotor shaft; while allowing a spiral convex zone to be disposed on the inner circumferential surface of the outer race member, provided on an outer circumferential surface of each of the planetary rollers is a circumferential groove system, which has the same pitch as the spiral convex zone and meshes with the spiral convex zone, or a spiral groove system, which has the same pitch as the spiral convex zone and a lead angle different from that of the spiral convex zone and meshes with the spiral convex zone; and the outer race member moves in an axial direction in response to revolving and orbital motion of each of the planetary rollers as a result of the rotation of the rotor shaft to thereby cause the brake pads to be linearly driven as a result of the axial movement of the outer race member. the electric linear actuator is configured such that a plurality of planetary rollers are interposed between an outer circumferential surface of a rotor shaft, which rotates integrally with a main shaft of the electric motor, and an inner circumferential surface of an outer race member that concentrically surrounds the rotor shaft, and each of the planetary rollers revolves while rotating around the rotor shaft as a result of the rotation of the rotor shaft; while allowing a spiral convex zone to be disposed on the inner circumferential surface of the outer race member, provided on an outer circumferential surface of each of the planetary rollers is a circumferential groove system, which has the same pitch as the spiral convex zone and meshes with the spiral convex zone, or a spiral groove system, which has the same pitch as the spiral convex zone and a lead angle different from that of the spiral convex zone and meshes with the spiral convex zone; and the outer race member moves in an axial direction in response to revolving and orbital motion of each of the planetary rollers as a result of the rotation of the rotor shaft to thereby cause the brake pads to be linearly driven as a result of the axial movement of the outer race member.