PROBLEM TO BE SOLVED: To rapidly and accurately move a boom to a desired position to hold the same at that position and to perform proper sprinkling fitted to the height of crops or the inclination of a field. SOLUTION: A spring built-in expansion cylindrical body 19 consisting of an inner cylindrical body housing one end of a compression spring in at least either one of a lower shaking arm 11 and an expansion drive means 15 interposed between a vertical shaft 5 and an upper shaking arm 10 or a boom support 32 to receive energizing force unidirectionally from the compression spring and an outer cylindrical body housing the other end of the compression spring and a part of the inner cylindrical body to receive energizing force from the compression spring in other direction is provided and an expansion drive means 23 fitted with the spring built-in expansion cylindrical body 19 connecting the acting part of the expansion drive means 23 applying driving force to the inner and outer cylindrical bodies is used.
