A gear device, comprising: an outer cylinder including an inner peripheral surface having a plurality of internal teeth formed thereon; 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, wherein the internal member includes: a gear (310, 320) having a plurality of external teeth meshed with the plurality of internal teeth; a carrier retaining a crank shaft assembly and configured to rotate relative to the outer cylinder about the rotation axis; a crank shaft assembly configured to cause the gear (310, 320) to perform oscillatory rotation so that a center of the gear (310, 320) revolves about the rotation axis, wherein the crank shaft assembly includes: a first journal retained by the carrier; a second journal retained by the carrier on an opposite side to the first journal; a first journal bearing configured to receive the first journal inserted thereinto; and a second journal bearing configured to receive the second journal inserted thereinto; 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 in cooperation with the first main bearing, wherein respective load action lines (FLL, SLL) of the first main bearing and the second main bearing pass through, in an extending direction of the rotation axis, an outer side of a rolling element of the first journal bearing and an outer side of a rolling element of the second journal bearing, respectively, and the outer cylinder includes a tubular case and an annular flange portion protruding outward from the case, characterized in that each of the load action line of the first main bearing and the load action line of the second main bearing passes through the case and the flange portion in a consecutive manner, and an intersection between the load action line of the first main bearing and the load action line of the second main bearing is positioned in the flange portion. an outer cylinder including an inner peripheral surface having a plurality of internal teeth formed thereon; 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, wherein the internal member includes: a gear (310, 320) having a plurality of external teeth meshed with the plurality of internal teeth; a carrier retaining a crank shaft assembly and configured to rotate relative to the outer cylinder about the rotation axis; a crank shaft assembly configured to cause the gear (310, 320) to perform oscillatory rotation so that a center of the gear (310, 320) revolves about the rotation axis, wherein the crank shaft assembly includes: a first journal retained by the carrier; a second journal retained by the carrier on an opposite side to the first journal; a first journal bearing configured to receive the first journal inserted thereinto; and a second journal bearing configured to receive the second journal inserted thereinto; a gear (310, 320) having a plurality of external teeth meshed with the plurality of internal teeth; a carrier retaining a crank shaft assembly and configured to rotate relative to the outer cylinder about the rotation axis; a crank shaft assembly configured to cause the gear (310, 320) to perform oscillatory rotation so that a center of the gear (310, 320) revolves about the rotation axis, wherein the crank shaft assembly includes: a first journal retained by the carrier; a second journal retained by the carrier on an opposite side to the first journal; a first journal bearing configured to receive the first journal inserted thereinto; and a second journal bearing configured to receive the second journal inserted thereinto; a first journal retained by the carrier; a second journal retained by the carrier on an opposite side to the first journal; a first journal bearing configured to receive the first journal inserted thereinto; and a second journal bearing configured to receive the second journal inserted thereinto; 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 in cooperation with the first main bearing, wherein respective load action lines (FLL, SLL) of the first main bearing and the second main bearing pass through, in an extending direction of the rotation axis, an outer side of a rolling element of the first journal bearing and an outer side of a rolling element of the second journal bearing, respectively, and the outer cylinder includes a tubular case and an annular flange portion protruding outward from the case, characterized in that each of the load action line of the first main bearing and the load action line of the second main bearing passes through the case and the flange portion in a consecutive manner, and an intersection between the load action line of the first main bearing and the load action line of the second main bearing is positioned in the flange portion.