PURPOSE: To make it possible to form a frame body without using any die member by composing a bearing frame body from an external frame body and an internal frame body of a square cylindrical pipe member, opposing an opening surface of the internal frame body to a wall surface of the external frame body and fixing both frame bodies with bolts each other.

CONSTITUTION: An external frame body 22 and an internal frame body 23 of a square cylindrical pipe member are such that an opening surface of the internal frame body 23 is opposed to a wall surface of the external frame body 22, and that both frame bodies are fixed with bolts 24 each other. The first shaft 39 is placed on the insides of the frame bodies 22 and 23 so as to correspond to mounting holes 27 and 28, and is held by bearing cylinders 41 and 42 welded to the internal frame body 23 through bushings 43 and 44 so that it is swiveling. The second shaft 40 is inserted in the first shaft 39 through a through hole 45, and is held so that is is capable of swiveling against the first shaft 39. A control lever 36 is inserted into an insertion groove 46 of the first shaft 39 so that it is capable of swiveling, and the second shaft 40 is inserted in the control lever 36. Oscillation of the control lever 36 around the first shaft 39 and the second shaft 40 is detected by potentiometers 63 and 66.