A new magnetometer was constructed using a high temperature superconducting bulk core (YBa₂Cu₃O₇-δ) in the liquid nitrogen. The permeability of bulk core changes with applying a coil-current magnetic fielcl or an externalmagndtic field due to flux penetration effect. Sensitive nonlinear inductance characteristics of high temperature superconducting bulk core with tightly wound coils was utilized for fluxgate magnetic-field sensors. The sensor is used for second-harmonic type.