This invention relates to linear excavation by a hydraulic excavator, such as a power shovel, having three service machines, a boom, an arm and a bucket. An oil pressure in an oil chamber on the boom-down side of a boom cylinder is introduced into a tank while kept at a predetermined set pressure at the time of linear excavation and the oil pressure on the boom-up side is led to the tank through a check valve for inhibiting the outflow of the oil so as to keep the boom under a so-called "float state" and to execute linear excavation by the operation of only the arm and the bucket. The present invention disposes also an automatic bucket driving system for driving and controlling automatically the bucket to a set bucket angle and operation this automatic bucket driving system at the time of linear excavation so as to further reduce the load on the operator.