To prevent each part from being damaged beforehand by improving the conditions of hydraulic oil.

Instead of a sealing circuit which connects the rod side pressure chambers 1b and 2b of two main hydraulic cylinders 1 and 2 to each other, the hydraulic drive circuit of a viscous fluid pump is provided with a tank 15 for storing hydraulic oil equivalent in flow amount to that in a sealing circuit, a hydraulic motor 16, and a hydraulic pump 17. Also the rod side pressure chambers 1b and 2b are connected to the hydraulic motor 16 through a line 18 with a check valve 20 and a line 19 with a check valve 21. The rod side pressure chambers 1b and 2b are connected to the delivery side of the hydraulic pump 17 through a line 22 with a check valve 24 and a line 23 with a check valve 25. In addition, hydraulic oil is fed from the tank 15 to the rod side pressure chambers 1b and 2b so as to replace the hydraulic oil with new one.