To provide a hydraulic control device of a work machine capable of maintaining a balance of working speed of both hydraulic actuators even though viscosity of working oil is increased in proportion to a decrease of working oil temperature and pressure loss of a hydraulic circuit is also increased during a combined operation of the two hydraulic actuators.

The hydraulic control device comprises a control valve 10 for merging pressurized oil supplied from a third hydraulic pump 3 with oil in a bottom supply pipe 32a of an arm cylinder 12, an electromagnetic proportional pressure-reducing valve 38 transmitting a switching signal Pc to the control valve 10, and a controller 50 calculating a driving current I to the electromagnetic proportional pressure reducing valve 38 on the basis of a boom raising operation signal Pb2 and operating oil temperature t. When the operating oil temperature is a designated temperature or lower, the control valve 10 is shut in proportion to decrease of the temperature t. Accordingly, work efficiency of combined operation using the two hydraulic actuators can be improved at cold temperature.