Hydraulic driving apparatus of work machine

Provided is a hydraulic driving apparatus capable of operating hydraulic actuators that actuate a boom, an arm, and a tip attachment at respective suitable speeds. The hydraulic driving apparatus includes a first hydraulic pump connected to a first main actuator, a second hydraulic pump connected to a second main actuator and an attachment actuator, a first merging selector valve that allows hydraulic fluid to be supplied from the first hydraulic pump to the second main actuator, and a power distribution control device that operates displacement of the first and second hydraulic pumps so as to decrease power distribution from a pump drive source to the second hydraulic pump and increase power distribution from the pump drive source to the first hydraulic pump when a specified combined operational action is performed on the second main actuator and the attachment actuator.