mounted to the tractor 2 by selective operations of the selector valves 30 there are three types of operations selectable i e one rendering both of the right and left accumulators 24 usable another rendering both unusable and still another rendering one of them usable rendering the other unusable first the case of rendering both of the right and left accumulators 24 usable is a case where during a work with vertically pivoting the boom members 8 by the boom cylinders 17 sudden change in the loads due to some cause is expected or it is necessary to avoid generation of such shock per se in such case the work will be carried out with opening the right and left selector valves 30 with this even if there occurs sudden surge in the oil pressure in the hydraulic pipes connected to the boom cylinders 17 this can be absorbed and relieved by the accumulators 24 thus effectively preventing transmission of significant shock to the driver and to the implement 9 such as the bucket as a result it is possible to avoid fall of the load as well as damage to the boom members 8 and the implement 9 moreover it is possible to avoid burst of the hydraulic pipes or oil leaks therefrom the case of rendering both the right and left accumulators 24 unusable is a case where the boom cylinders 17 need to be operated with maximum hydraulic forces in such case the work will be effected with closing the right and left selector.