To provide a hydraulic drive system of a wheel type working machine that can be started on the slope without moving down.

This machine includes: a selector switch 26 which selects an operation brake mode as a temporary stop mode of a traveling body when a working device is in operation; a proportional solenoid valve 27 for controlling a direction control valve for braking; and a controller for outputting a braking operation signal S1 or a brake release signal S2 to the proportional solenoid valve 27 according to a signal output from a pressure sensor 24 for a braking manipulated variable. The controller includes a delay means that delays the brake release signal S2 according to the signal output from a pressure sensor 18 for a running manipulated variable, outputs the brake release signal S2 delayed by the delay means to the proportional solenoid valve 27.