A SQUID system, such as a SQUID magnetometer, including a SQUID and a bias circuit supply a pulsed bias current to cause an output of a pulsed output from an injection point in the SQUID. The SQUID has a predetermined threshold characteristic which is asymmetrical with respect to a coordinate defined by a current to be measured and flowing through a first junction line and the bias current. The bias current supplied from the bias circuit comprises pulses which are alternately positive polarity pulses and negative polarity pulses, and has a first frequency. The threshold characteristic and the bias current are defined to output an output signal comprising pulses, the number thereof being proportional to the current to be measured and a polarity thereof corresponding to a polarity of the current to be measured.