In case of cooling a SQUID magnetometer by a cryogenic refrigerator, a tubular resinous bobbin of the SQUID magnetometer around which a pick-up coil is wound is arranged so as to have wires, in the body of the bobbin, which are each coated with a resinous film, made of non-magnetic material with high thermal conductivity such as copper, and netted in a grid pattern substantially in an axial direction of the bobbin and a circumferential direction of the bobbin. According to the arrangement, thermal conduction characteristic of the bobbin is improved, thereby enhancing efficiency of cooling the pick-up coil made of a superconducting wire. This accomplishes an effective cooling of the SQUID magnetometer by the refrigerator.