PURPOSE: To improve sealability of a free piston and to readily know behavior of the free piston by laying a pressurizing rod passing through the side of a material to be treated of a high pressure container at the side of the material to be treated of the free piston.

CONSTITUTION: In a free piston type pressure treatment device in which a high pressure container 2 is divided into two liquid chambers 6 and 7 by a free piston 5 movable in its shaft direction and the fluid in one fluid chamber 6 is pressurized to pressurize a material to be treated in the other fluid chamber T through the free piston 5, the end of a pressurizing rod 15 passing in the shaft direction through the side of the other fluid chamber 7 of the high pressure container 2 is fixed to the face of the other fluid chamber 7 of the free piston 5.