PURPOSE: To balance an opening and closing member in an arbitrary position by the friction force and the reaction force of both arm parts by winding the helical part of a torsion spring compressively deformably in an axial direction, sandwiching the same from both sides by means of friction plates, and connecting the one part of both arm parts of said spring to the opening and closing member and the other to the machine frame side.

CONSTITUTION: A torsion spring 1 has a winding part 1a of a helical shape, and said part 1a consists of plural coil springs. Said part 1a is grasped axially from both sides by means of an axial supporter 2, a washer 3 and a screw 4, and both arm parts 1b, 1c are connected respectively to an opening and closing member and a machine frame. In this way, an automatic balance stay is constituted, and the automatic balancing of the opening and closing member in an arbitrary position of full opening and closing angles is made possible by the compression force of the spring 1a and the torsion reaction force of the parts 1b, 1c.