A speed reduction mechanism, comprising: a rotary shaft with an eccentric portion (42a, 42b); an input member (50, 51) that is an external gear rotatably arranged on an outer periphery of the eccentric portion of the rotary shaft and having a plurality of through-holes (50b, 51b) arranged around an axis (O 3, O 2, O' 3, O' 2) of the input member (50, 51) at equal angular intervals; a rotation force applying member that is an internal gear meshed with the input member (50, 51) and having teeth of which the number is larger than the number of teeth of the external gear; an output mechanism having a plurality of output members that receive rotation force applied to the input member (50, 51) by the rotation force applying member and output the rotation force to an output target as torque of the output target, and that are passed through the respective through-holes; and a bearing mechanism arranged on an axis of the output mechanism and including a pair of rolling bearings (34, 66) that are arranged in parallel with each other in an axial direction of the output target, wherein the output target is rotatably supported by the rolling bearings (34, 66) of the bearing mechanism such that an outer periphery of one of the rolling bearings (34, 66) faces an inner periphery of a housing that accommodates the output target, and at least part of an outer periphery of the other one of the rolling bearings (34, 66) faces an inner periphery of the rotation force applying member, characterized in that the housing is made of a material having a stiffness lower than a stiffness of a material of the rotation force applying member. a rotary shaft with an eccentric portion (42a, 42b); an input member (50, 51) that is an external gear rotatably arranged on an outer periphery of the eccentric portion of the rotary shaft and having a plurality of through-holes (50b, 51b) arranged around an axis (O 3, O 2, O' 3, O' 2) of the input member (50, 51) at equal angular intervals; a rotation force applying member that is an internal gear meshed with the input member (50, 51) and having teeth of which the number is larger than the number of teeth of the external gear; an output mechanism having a plurality of output members that receive rotation force applied to the input member (50, 51) by the rotation force applying member and output the rotation force to an output target as torque of the output target, and that are passed through the respective through-holes; and a bearing mechanism arranged on an axis of the output mechanism and including a pair of rolling bearings (34, 66) that are arranged in parallel with each other in an axial direction of the output target, wherein the output target is rotatably supported by the rolling bearings (34, 66) of the bearing mechanism such that an outer periphery of one of the rolling bearings (34, 66) faces an inner periphery of a housing that accommodates the output target, and at least part of an outer periphery of the other one of the rolling bearings (34, 66) faces an inner periphery of the rotation force applying member, characterized in that the housing is made of a material having a stiffness lower than a stiffness of a material of the rotation force applying member.