PURPOSE: To correctly regulate a relative position of a pickup coil with respect to a vacuum container by interposing a thermal anchor which has high thermal conductivity and is not superconductive even at an extremely low temperature level.

CONSTITUTION: A pickup coil 2b is held at a tip 1f of a vacuum container via a heat insulation layer 1g. By airtightly coupling the tip to a vacuum container part 1b in this state, good thermal coupling between a final cooling stage 1e and the coil 2b via a thermal anchor 2c can be attained. In addition since the coil 2b is pressed against an upper bottom of the tip if by means of spring property of the anchor 2c, correct positioning with respect to the tip 1f can be attained. Therefore without subsequently performing task of estimating the position of the coil 2b using an external coil or the like, (eliminating need for operating a freezer to obtain a superconductive transition temperature which is needed to perform the above task), correct measurement of a living body magnetic field or the like can be performed based on position information of the coil 2b which has been obtained at the time of assembly.