HYDRAULIC CIRCUIT OF HYDRAULIC SHOVEL

PROBLEM TO BE SOLVED: To make it possible to increase working speed of composite work necessary for a large amount of driving pressure oil for an arm while keeping the wasteful consumption of fuel down to a minimum, while a large amount of pressure oil is not necessary without imposing so large load pressure on the boom like excavation work, etc. SOLUTION: Based on pilot pressure Pp from an arm switching pilot valve 22 detected by a pilot pressure sensor 34, a controller 33 outputs a branch flow control signal having value obtained by performing an operation based on a transformation function prescribing relation between the pilot pressure Pp read out from a storage section and target opening area ST of an auxiliary selector valve 23 and a plurality of transformation functions into a proportional electromagnetic valve 32. The opening area SS of the auxiliary selector valve 23 is controlled by pilot oil flowing out from the proportional electromagnetic valve 32, and when the discharge pressure Pp is in excess of lower limited discharge pressure P0 , branch flow of driving pressure oil flowing out to oil supply side of a combined flow selector valve 17 passing through an alternative bypass oil passage is increased with the transformation function to control.