PURPOSE: To realize a space wherein a magnetic field is extremely low by providing a magnetic flux producing means which impresses a magnetic field to the external surface or internal surface of a shield body and forming a low critical current area whose critical current is lower than those of other parts at part of the shield body.

CONSTITUTION: On the external surface of the shield body 1, a coil 2 and a power source 3 for applying the magnetic field are provided. The coil 2 and power source 3 constitute the magnetic flux producing means. Further, the low critical current area 1c is formed at part of the shield body 1 and the superconductor atop of the projection part 1b of the shield body 1 is make a half as thick as at other parts to reduce the critical current value of the part. Thus, the critical current area 1c is provided at part of the shield body 1 to specify the place where magnetic flux is trapped and the extremely low magnetic field is realized in other internal space over a wide range.