power train shaft 53 which is connected to the operating power take off shaft 52 on the front side of the carriage and an input shaft 35 in which a bevel gear 34 that meshes with the bevel gear 33 is provided to the end of the shaft a boss section 41 a of an input flat gear 41 fitted over a spline formed on the portion of the power input shaft 35 that projects from the gear box 32 is supported by a shaft via ball bearings 44 in an intermediate section of the connecting casing 31 and an odd number of relaying flat gears 43 i e a single such gear is disposed between the input flat gear 41 and the output flat gear 42 fitted over a spline on the shaft ends of one of the blade shafts 22 an even number of flat gears 43 i e two such gears are disposed between the input flat gear 41 and the output flat gear 42 fitted over a spline on the end of the other blade shaft 22 the flat gears 40 that constitute the power train 4 are composed of the flat gears 41 42 and 43 the right and left blades 21 that are connected and interlocked using the power train 4 which is composed of the flat gears 40 in this manner are disposed so that the rotating.