Abstract: A superconducting quantum interference fluxmeter, a super-high-sensitivity fluxmeter using a SQUID, suitable for use in precision electric instrumentation and superconducting electronics. The fluxmeter comprises a system to detect response output from the SQUID with high sensitivity by applying thereto such exciting AC as has a plurality of frequency components from an excitation AC source, and a system to detect with high sensitivity such alternating current as has higher or lower harmonic component of the exciting AC, which is generated by utilizing electromagnetic nonlinearity of the SQUID. Thus this invention markedly relieves difficulties in the manufacture and the use of the superconducting quantum interference devices, and enables selection of any desired operating temperature range, and compensation of quality changes with time.