held at the neutral positions respectively further the operation stick 28 is provided with shaft portions 31 protrusively provided at the left and right portions of the lower portion of the operation stick 28 as shown in fig2 and 4 as shown in fig2 and 6 the lower arm 23 has the side wall portions 23 a formed by the long hole 27 bearing portions 32 are formed at the lower faces of the center portions of the left and right side wall portions 23 a in almost recessed semicircular shapes respectively thus the bearing portions 32 are provided in a manner that the lower faces thereof are opened at the side wall portions 23 a of the lower arm 23 respectively further the bearing portions 32 are formed to have sizes corresponding to the sizes of the shaft portions 31 which are provided at the operation stick 28 so that the shaft portions are fitted in loosely in the bearing portions furthermore the left and right shaft portions 31 are formed to have different sizes and the bearing portions 32 corresponding thereto also are formed to have different sizes in this case in a case where the left and right shaft portions 31 are fitted in from the lower faces of the bearing portions 32 if the shaft portions are mistakenly fitted in loosely in erroneous.