PURPOSE: To feed material having low fluidity and high viscosity in a pressure feeding unit for a fluid such as sludge or the like by thrusting a slide cylinder into a fluid in a hopper to take the fluid in the cylinder and pushing out the fluid by a sliding motion of a piston.

CONSTITUTION: A slide cylinder 2 is retreated outside a hopper 1 to position a piston 3 at a boundary between the slide cylinder 2 and the hopper 1 and fill the hopper 1 with concrete not hardened. Subsequently, the slide cylinder 2 is pressed into the concrete not hardened by a cylinder 23 to take the concrete not hardened in the slide cylinder 2. A seal ring 21 mounted at the forward end of the slide cylinder 2 is pressed to a ring holder of a discharge port 13 to stop the slide cylinder 2, and a discharge valve 14 is opened to forward the piston 3, thereby pushing out the concrete not hardened in the slide cylinder 2. After that, the slide cylinder 2 and the piston 3 are retreated, and the forwarding and pushing operations are repeated. Thus, low-fluidity and high-viscosity material can be fed by pressure efficiently.