To reduce noise and erroneous function by interrupting source current supply to a read circuit when a comparator is making a decision whether a signal from an SQUID is present or not.

Output signal from an SQUID 11 is passed through a load inductance and a load resistor and inputted, as a magnetic signal, to a comparator 12 comprising a Josephson element which then outputs the variation of inductance to a read circuit 13. In this regard, the comparator 12 makes a decision whether a signal from the SQUID 11 is present or not and if a signal is present, and the circuit 13 is notified that the signal from the SQUID 11 is 1 when a source current is fed. If no signal is present, no flux is inputted even if the source current is fed and since no voltage is generated, the circuit 13 is notified that the signal from the SQUID 11 is 0. Since no source current is fed to the circuit 13 when the comparator 12 is making a decision, the comparator 12 is not susceptible to the noise generated from other circuit and thereby erroneous function is reduced.