A robot arm attachable to a base frame, including: a link member of which one end is rotatable with three degrees of freedom with respect to the base frame; and a second rotary link which is connected to the other end of the link member and is rotatable with two degrees of freedom with respect to the link member, wherein: the link member is rotatable with respect to the base frame in a first rotation about a first rotary shaft via a first rotary member, in a second rotation about a second rotary shaft, and in a third rotation about a third rotary shaft, the second rotary shaft and the third rotary shaft intersecting each other, and the second rotary link is rotatable with respect to the link member about a fourth rotary shaft and a fifth rotary shaft intersecting each other, wherein the first rotation of the link member with respect to the base frame is a yaw rotation aligned with the direction of the gravity, and the second and third rotations of the link member with respect to the base frame are respectively a roll rotation and a pitch rotation, wherein the second and third rotations of the link member are restrained by a first differential bevel gear wherein a pair of first cam plates is fixed to shafts of a pair of first rotary bevel gears of the first differential bevel gear, and a one-degree-of-freedom weight compensator is provided to be connected to the first cam plates and wherein the rotations of the second rotary link with respect to the link member are restrained by the second and third rotations of the link member so that the second rotary link is operated in parallel to the first rotary member. a link member of which one end is rotatable with three degrees of freedom with respect to the base frame; and a second rotary link which is connected to the other end of the link member and is rotatable with two degrees of freedom with respect to the link member, wherein: the link member is rotatable with respect to the base frame in a first rotation about a first rotary shaft via a first rotary member, in a second rotation about a second rotary shaft, and in a third rotation about a third rotary shaft, the second rotary shaft and the third rotary shaft intersecting each other, and the second rotary link is rotatable with respect to the link member about a fourth rotary shaft and a fifth rotary shaft intersecting each other, wherein the first rotation of the link member with respect to the base frame is a yaw rotation aligned with the direction of the gravity, and the second and third rotations of the link member with respect to the base frame are respectively a roll rotation and a pitch rotation, wherein the second and third rotations of the link member are restrained by a first differential bevel gear wherein a pair of first cam plates is fixed to shafts of a pair of first rotary bevel gears of the first differential bevel gear, and a one-degree-of-freedom weight compensator is provided to be connected to the first cam plates and wherein the rotations of the second rotary link with respect to the link member are restrained by the second and third rotations of the link member so that the second rotary link is operated in parallel to the first rotary member. the link member is rotatable with respect to the base frame in a first rotation about a first rotary shaft via a first rotary member, in a second rotation about a second rotary shaft, and in a third rotation about a third rotary shaft, the second rotary shaft and the third rotary shaft intersecting each other, and the second rotary link is rotatable with respect to the link member about a fourth rotary shaft and a fifth rotary shaft intersecting each other, wherein the first rotation of the link member with respect to the base frame is a yaw rotation aligned with the direction of the gravity, and the second and third rotations of the link member with respect to the base frame are respectively a roll rotation and a pitch rotation, wherein the second and third rotations of the link member are restrained by a first differential bevel gear wherein a pair of first cam plates is fixed to shafts of a pair of first rotary bevel gears of the first differential bevel gear, and a one-degree-of-freedom weight compensator is provided to be connected to the first cam plates and wherein the rotations of the second rotary link with respect to the link member are restrained by the second and third rotations of the link member so that the second rotary link is operated in parallel to the first rotary member.