A rotational structure comprising: a base member; a shaft member including one end fixed to the base member, and extending in an axial direction of the rotational structure; a rotational member attached to the shaft member through a bearing, and rotatable about the shaft member; an encoder configured to detect a rotation angle of the rotational member, and an annular thrust bearing provided to surround the shaft member, between the base member and the rotational member, wherein a hollow portion, in which the base member is opposed to the rotational member, is formed around the shaft member inside the rotational structure and between an internal circumferential surface of the thrust bearing and an external circumferential surface of the shaft member, the encoder includes: a detection target member provided around the shaft member in the hollow portion such that the detection target member is rotated together with one of the rotational member and the base member, and having a physical quantity changing in a circumferential direction; and a detector capable of detecting the physical quantity of the detection target member, and provided at a position in the hollow portion where the physical quantity of the detection target member is detectable, such that the detector is rotated together with the other of the rotational member and the base member, wherein the detection target member and the detector are opposed to each other in the axial direction, and a space between the detection target member and the detector overlaps a range in which the thrust bearing is provided, in the axial direction. a base member; a shaft member including one end fixed to the base member, and extending in an axial direction of the rotational structure; a rotational member attached to the shaft member through a bearing, and rotatable about the shaft member; an encoder configured to detect a rotation angle of the rotational member, and an annular thrust bearing provided to surround the shaft member, between the base member and the rotational member, wherein a hollow portion, in which the base member is opposed to the rotational member, is formed around the shaft member inside the rotational structure and between an internal circumferential surface of the thrust bearing and an external circumferential surface of the shaft member, the encoder includes: a detection target member provided around the shaft member in the hollow portion such that the detection target member is rotated together with one of the rotational member and the base member, and having a physical quantity changing in a circumferential direction; and a detector capable of detecting the physical quantity of the detection target member, and provided at a position in the hollow portion where the physical quantity of the detection target member is detectable, such that the detector is rotated together with the other of the rotational member and the base member, wherein the detection target member and the detector are opposed to each other in the axial direction, and a space between the detection target member and the detector overlaps a range in which the thrust bearing is provided, in the axial direction. a detection target member provided around the shaft member in the hollow portion such that the detection target member is rotated together with one of the rotational member and the base member, and having a physical quantity changing in a circumferential direction; and a detector capable of detecting the physical quantity of the detection target member, and provided at a position in the hollow portion where the physical quantity of the detection target member is detectable, such that the detector is rotated together with the other of the rotational member and the base member, wherein the detection target member and the detector are opposed to each other in the axial direction, and a space between the detection target member and the detector overlaps a range in which the thrust bearing is provided, in the axial direction.