A gear device (1; 101) for transmitting torque, at a predetermined reduction gear ratio, between a pair of partner members (50, 52), comprising: an outer cylinder configured to be fixed to one of the pair of partner members (50, 52); a plurality of crank shafts; a carrier configured to be fixed to the other one of the pair of partner members (50, 52) 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 (50, 52); a plurality of crank shafts; a carrier configured to be fixed to the other one of the pair of partner members (50, 52) 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.