as shown in fig2 setting the machine proper 10 at a distance from the trench line a turning the turn base 14 to put the tip of the boom 16 on the trench line a and finally bringing the arm 22 and the bucket 28 onto the trench line a since the second turning shaft 34 is provided at the tip of the boom 16 the intercentral distance l between the trench line a and the first turning shaft p may be made considerably long as seen from fig2 which satisfies one of favorable conditions for digging a trench the structure that the second turning shaft 34 is kept vertical leads to an advantage that both the arm 22 and the bucket 28 are operated in the vertical plane over the trench line a i e that the jobs of excavation delivery and removal are carried out with both the arm 22 and the bucket 28 kept vertical to the ground surface it follows that since the blade of the bucket 28 is kept flat with respect to the ground surface the bottom face of a trench may be made horizontal and smooth the corner section is made to be at a good right angle and that the bucket 28 will not go aslant on the side wall of trench resulting in a vertical wall face even when the arm 22 is.