A gear device (1; 101) for transmitting torque, at a predetermined reduction gear ratio, between a pair of partner members , comprising: an outer cylinder configured to be fixed to one of the pair of partner members ; a plurality of crank shafts ; a carrier configured to be fixed to the other one of the pair of partner members and rotatably housed inside the outer cylinder , and rotatably supports the plurality of crank shafts , and rotating relative to the outer cylinder in synchronization with the rotation of the plurality of crank shafts (20; a plurality of spur gears in which each of the spur gears is fixed coaxially to each of the crank shafts at a portion of each of the crank shafts extending toward an outer side of the carrier ; and an input gear which includes a gear part that engages with the plurality of spur gears , and transmits rotational drive force to each of the plurality of spur gears ; characterized in that washer means comprising a plurality of washers (41, 42; 141, 142) being disposed outside the carrier , the input gear being regulated from moving in a radial direction of the input gear by engaging with the plurality of spur gears at a position outside the carrier in an axial direction of the carrier , and regulated from moving in an axial direction of the input gear by the washer means; the gear device (1; 101) further includes snap rings (43, 44; 143, 144) which removably mount the spur gear in a state of being sandwiched by the pair of washers (41, 42; 141, 142) on the crank shaft , each of the plurality of washers (41, 42; 141, 142) is disposed separately on either side of each of the spur gears , and the snap rings (43, 44; 143, 144) regulate the spur gear and the pair of washers (41, 42; 141, 142) from moving in an axial direction of the crank shafts . an outer cylinder configured to be fixed to one of the pair of partner members ; a plurality of crank shafts ; a carrier configured to be fixed to the other one of the pair of partner members and rotatably housed inside the outer cylinder , and rotatably supports the plurality of crank shafts , and rotating relative to the outer cylinder in synchronization with the rotation of the plurality of crank shafts (20; a plurality of spur gears in which each of the spur gears is fixed coaxially to each of the crank shafts at a portion of each of the crank shafts extending toward an outer side of the carrier ; and an input gear which includes a gear part that engages with the plurality of spur gears , and transmits rotational drive force to each of the plurality of spur gears ; characterized in that washer means comprising a plurality of washers (41, 42; 141, 142) being disposed outside the carrier , the input gear being regulated from moving in a radial direction of the input gear by engaging with the plurality of spur gears at a position outside the carrier in an axial direction of the carrier , and regulated from moving in an axial direction of the input gear by the washer means; the gear device (1; 101) further includes snap rings (43, 44; 143, 144) which removably mount the spur gear in a state of being sandwiched by the pair of washers (41, 42; 141, 142) on the crank shaft , each of the plurality of washers (41, 42; 141, 142) is disposed separately on either side of each of the spur gears , and the snap rings (43, 44; 143, 144) regulate the spur gear and the pair of washers (41, 42; 141, 142) from moving in an axial direction of the crank shafts . <EOS>