A hydraulic drive device for a work vehicle, comprising: a main pump of a variable displacement type or a fixed displacement type, the main pump discharging pressure oil; a main flow passage for supplying pressure oil of the main pump to an actuator; a sub-pump of a fixed displacement type discharging pressure oil; a sub-flow passage for making pressure oil of the sub-pump merge with the main flow passage and supplying the pressure oil to the actuator; a merging directional valve (6A, 6B, 6C) for connecting or cutting off the main flow passage and the sub-flow passage; a controller for controlling operation of the merging directional valve (6A, 6B, 6C); a relief valve (7A, 7B) having a relief pressure for returning pressure oil flowing through the sub-flow passage to a tank; and a return flow passage that connects the merging directional valve (6A, 6B, 6C) and the tank, wherein the relief valve (7A, 7B) has a pressure override characteristic having a tendency that the relief pressure increases from a cracking pressure to a set pressure as a relief flow rate increases, wherein the sub-flow passage and the return flow passage communicate with each other through the merging directional valve (6A, 6B, 6C) in case the merging directional valve (6A, 6B, 6C) operates and the main flow passage and the sub-flow passage are cut off, characterized in that a pressure sensor detects the pressure of the sub-flow passage, and the controller operates the merging directional valve (6A, 6B, 6C) and cuts off the main flow passage and the sub-flow passage in case a predetermined time elapses after a pressure signal inputted from the pressure sensor reaches the cracking pressure, the predetermined time being the time when the pressure of the sub-flow passage rises from the cracking pressure to the set pressure, or in case the pressure signal inputted from the pressure sensor reaches the set pressure. a main pump of a variable displacement type or a fixed displacement type, the main pump discharging pressure oil; a main flow passage for supplying pressure oil of the main pump to an actuator; a sub-pump of a fixed displacement type discharging pressure oil; a sub-flow passage for making pressure oil of the sub-pump merge with the main flow passage and supplying the pressure oil to the actuator; a merging directional valve (6A, 6B, 6C) for connecting or cutting off the main flow passage and the sub-flow passage; a controller for controlling operation of the merging directional valve (6A, 6B, 6C); a relief valve (7A, 7B) having a relief pressure for returning pressure oil flowing through the sub-flow passage to a tank; and a return flow passage that connects the merging directional valve (6A, 6B, 6C) and the tank, wherein the relief valve (7A, 7B) has a pressure override characteristic having a tendency that the relief pressure increases from a cracking pressure to a set pressure as a relief flow rate increases, wherein the sub-flow passage and the return flow passage communicate with each other through the merging directional valve (6A, 6B, 6C) in case the merging directional valve (6A, 6B, 6C) operates and the main flow passage and the sub-flow passage are cut off, characterized in that a pressure sensor detects the pressure of the sub-flow passage, and the controller operates the merging directional valve (6A, 6B, 6C) and cuts off the main flow passage and the sub-flow passage in case a predetermined time elapses after a pressure signal inputted from the pressure sensor reaches the cracking pressure, the predetermined time being the time when the pressure of the sub-flow passage rises from the cracking pressure to the set pressure, or in case the pressure signal inputted from the pressure sensor reaches the set pressure.