A joint structure is employed for an articulated type robot in which a speed reducer of one-stage speed reducing structure and a servo motor are directly connected to each other for the joint of the rotary base B1 and the lower arm A1 and for the joint of the lower arm A1 and the upper arm A2. The input shaft 2 of the speed reducer 10 directly connected to the output shaft of the servo motor 1 attached to the first member 8 (B1/A2) is a crank shaft composing a cam, and the external gears 3a, 3b, 3c are meshed with the cam surfaces. When the servo motor 1 is driven and the input shaft 2 is rotated, the external gears 3a, 3b, 3c are eccentrically revolved and rotated while the external gears 3a, 3b, 3c are meshed with the internal gear 4 provided inside the case 5 of the speed reducer 10. The rotation is taken out by a plurality of pin members 6 engaged to the external gears 3a, 3b, 3c and transmitted to the output shaft 7 of the speed reducer 10. Therefore, the second member 9 (A1) is relatively pivoted with respect to the first member 8. Due to the foregoing, the robot structure in which a plurality of joint devices for connecting the adjoining links via the speed reducer are connected in series to each other can be simplified.