A rotary actuator (1, 1A) comprising: one hollow motor; two hollow reduction gears (3, 4) having a same reduction ratio; and one rotary shaft, wherein the hollow reduction gears (3, 4) are coaxially arranged on both sides of the hollow motor in a direction of a center axis line to sandwich the hollow motor therebetween; the hollow motor has: a hollow motor shaft; and a motor hollow part that is defined by a hollow part of the hollow motor shaft and extends to penetrate the hollow motor in the direction of the center axis line; each of the hollow reduction gears (3, 4) has: a hollow input shaft (33, 43); a hollow output shaft (34, 44); and a reduction gear hollow part that is defined by a hollow part of the hollow input shaft (33, 43) and extends to penetrate the hollow reduction gear (3, 4) in the direction of the center axis line; the rotary shaft coaxially extends to penetrate the motor hollow part; the hollow input shafts (33, 43) of the hollow reduction gears (3, 4) are coaxially connected to shaft end parts (22b, 22c) on both sides of the hollow motor shaft, respectively; the rotary shaft has shaft portions (51, 52) on both sides thereof protruding from both ends of the motor hollow part, characterized in that the hollow output shafts (34, 44) of the hollow reduction gears (3, 4) are coaxially connected to the shaft portions via connection mechanisms (6, 7); and at least one of the connection mechanisms (6, 7)) has an adjustment part to adjust a connection position of the hollow output shaft (34, 44) to the rotary shaft by a predetermined angle in a rotational direction of the hollow output shaft (34, 44). one hollow motor; two hollow reduction gears (3, 4) having a same reduction ratio; and one rotary shaft, wherein the hollow reduction gears (3, 4) are coaxially arranged on both sides of the hollow motor in a direction of a center axis line to sandwich the hollow motor therebetween; the hollow motor has: a hollow motor shaft; and a motor hollow part that is defined by a hollow part of the hollow motor shaft and extends to penetrate the hollow motor in the direction of the center axis line; each of the hollow reduction gears (3, 4) has: a hollow input shaft (33, 43); a hollow output shaft (34, 44); and a reduction gear hollow part that is defined by a hollow part of the hollow input shaft (33, 43) and extends to penetrate the hollow reduction gear (3, 4) in the direction of the center axis line; the rotary shaft coaxially extends to penetrate the motor hollow part; the hollow input shafts (33, 43) of the hollow reduction gears (3, 4) are coaxially connected to shaft end parts (22b, 22c) on both sides of the hollow motor shaft, respectively; the rotary shaft has shaft portions (51, 52) on both sides thereof protruding from both ends of the motor hollow part, characterized in that the hollow output shafts (34, 44) of the hollow reduction gears (3, 4) are coaxially connected to the shaft portions via connection mechanisms (6, 7); and at least one of the connection mechanisms (6, 7)) has an adjustment part to adjust a connection position of the hollow output shaft (34, 44) to the rotary shaft by a predetermined angle in a rotational direction of the hollow output shaft (34, 44).