A gear device , comprising: an outer cylinder including a tubular case and an annular flange portion protruding outward from the case ; an internal member at least partially housed in the outer cylinder and configured to rotate relative to the outer cylinder about a predetermined rotation axis (RAX); a first main bearing fitted into an annular space formed between the outer cylinder and the internal member ; and a second main bearing fitted into the annular space and configured to define the rotation axis (RAX) in cooperation with the first main bearing , wherein a distance between a first intersection (FIP) where a load action line (FLL) of the first main bearing intersects with the rotation axis (RAX) and a second intersection (SIP) where a load action line (SLL) of the second main bearing intersects with the rotation axis (RAX) is set to fall within a range expressed by: BBD + BRD × 2 × 0.95 < IID < BBD + BRD × 2 × 1.05 IID : Distance Between First Intersection and Second Intersection BBD : Distance Between First Main Bearing and Second Main Bearing BRD: Distance Between First Main Bearing and Rotation Axis characterized in that each of the load action line (FLL) of the first main bearing and the load action line (SLL) of the second main bearing passes through the case and the flange portion in a consecutive manner, and an intersection (ISP) between the load action line (FLL) of the first main bearing and the load action line (SLL) of the second main bearing is positioned in the flange portion . an outer cylinder including a tubular case and an annular flange portion protruding outward from the case ; an internal member at least partially housed in the outer cylinder and configured to rotate relative to the outer cylinder about a predetermined rotation axis (RAX); a first main bearing fitted into an annular space formed between the outer cylinder and the internal member ; and a second main bearing fitted into the annular space and configured to define the rotation axis (RAX) in cooperation with the first main bearing , wherein a distance between a first intersection (FIP) where a load action line (FLL) of the first main bearing intersects with the rotation axis (RAX) and a second intersection (SIP) where a load action line (SLL) of the second main bearing intersects with the rotation axis (RAX) is set to fall within a range expressed by: BBD + BRD × 2 × 0.95 < IID < BBD + BRD × 2 × 1.05 IID : Distance Between First Intersection and Second Intersection BBD : Distance Between First Main Bearing and Second Main Bearing BRD: Distance Between First Main Bearing and Rotation Axis IID : Distance Between First Intersection and Second Intersection BBD : Distance Between First Main Bearing and Second Main Bearing BRD: Distance Between First Main Bearing and Rotation Axis characterized in that each of the load action line (FLL) of the first main bearing and the load action line (SLL) of the second main bearing passes through the case and the flange portion in a consecutive manner, and an intersection (ISP) between the load action line (FLL) of the first main bearing and the load action line (SLL) of the second main bearing is positioned in the flange portion . <EOS>