To provide a hydraulic drive unit of a working machine capable of reducing hydraulic piping by abolishing a pilot hydraulic pressure source.

A hydraulic drive unit 2u is equipped with a turning motor (hydraulic actuator) 12 for rotatably driving a turning body (driven body) 2 in both forward and reverse directions, a first and second supply/discharge passages 12a, 12b for supplying/discharging working oil to/from the turning motor 12, a control valve 12c for switching a flow of the working oil with respect to the first and second supply/discharge passages 12a, 12b, wherein operation of the control valve 12c is controlled by drive current output from a controller 19. The unit is equipped with a hydraulic brake 22 for releasing the braking of the turning body 2 by increasing oil pressure to be supplied while braking the turning body 2 in a state that oil pressure to be supplied is low, a high-pressure selecting valve (high-pressure selecting means) 32 for selectively communicating with the high-pressure sides of the first and second supply/discharge passages 12a, 12b, and a break releasing pressure passage 52 for leading to the hydraulic brake 22 the working oil pressure taken out through the high-pressure selecting valve 32. The operation of the hydraulic valve 22 is released in association with the operation of the turning motor 12.