PURPOSE: To improve safety as well as to miniaturize a position detecting sensor by arranging the position detecting sensor in a rotary joint part in an arm detecting mechanism for an articulated robot, for detecting the rotating position on the rotating side in relation to a rotary shaft.

CONSTITUTION: A shoulder joint part 13 and an elbow joint part 14 rotatably supporting rotational arms 11, 12 by rotary shafts 18, 19 are provided on an articulated robot. Origin detecting sensors 31 and limit detecting sensors 32, 33 for detecting the rotating positions of the rotating arms 11, 12 are arranged in a rotating arm 11 so that detecting screws 33a may be operated in the circumferential directions of the rotary shafts 18, 19 and may be screwed to supporting rings 36 to 39 capable of being fixed on the peripheral surface, and so that proximity sensors 33b may face the screws 33a on the coaxial peripheral flat surface taking the axes of the rotary shafts 18, 19 as the center.