in fig3 there is interposed between the blade 20 and the u shaped frame 22 a coupling means 25 having a spherical shaft 26 fixedly secured through a bracket to the blade 20 and a cap 28 for mounting the spherical shaft 26 onto the leading end of the above mentioned shaft 24 by means of a plurality of bolts 30 in such a manner that the spherical shaft 26 may be turned freely in all directions to thereby enable the blade 20 to turn freely in all directions relative to the shaft 24 whilst the rear ends of both the leg portions of the u shaped frame 22 which extend in parallel relationship with each other are pivotally mounted on trunnion shafts 32 and 34 respectively which are each supported through a bracket fixedly secured to the vehicle body further left and right arms 36 and 38 are each connected through a bracket to the blade 20 at two places on the rear surface thereof in such a manner that one end of each of the arms may be turned freely said two places corresponding to left and right positions symmetrically of the longitudinally central portion of the blade 20 at a height.