PURPOSE: To raise the work efficiency of a hoop wrench, by operating a bending arm of a winding hoop by a piston rod by oil pressure, when the winding hoop is wound round a main hoop by the hoop wrench in case of reinforced concrete construction.

CONSTITUTION: The end part of a winding hoop H to be wound to a main hoop S is placed at a prescribed position so as to fall at a right angle with the main hoop S. A main hoop receiving face 45a of a hoop wrench is made to press- contact with the main hoop S, and the winding hoop H is placed against the main hoop receiving face 45a by an arm 41. Subsequently, a cam 16 is rotated by driving a motor 11, by which a piston 17 is made to perform a reciprocating motion, oil in an oil tank 14 is compressed and pressurized oil is obtained. The pressurized oil goes into a cylinder chamber 20, moves a piston rod 38 to the left, the arm 41 approaches a main hoop receiving part 45, the end part of the winding hoop H is bent like H" and it is wound around the main hoop S. In this way, the winding hoop can be wound around the main hoop by simple handling and with high work efficiency.