Provided is a magnetoimpedance (MI) sensor having a high magnetic sensor sensitivity and a wide measurement range. The MI sensor comprises an MI element, an electric current supply unit and a signal processing circuit. The MI element comprises a magnetosensitive wire formed of an amorphous soft magnetic alloy having zero magnetostriction, and a detection coil provided around the magnetosensitive wire with an electric insulator disposed therebetween, thereby detecting voltage generated at the detection coil and corresponding to an external magnetic field upon application of a high frequency electric current to the magnetosensitive wire. The electric current supply unit supplies the high frequency electric current to the MI element. The signal processing circuit processes an output signal from the detection coil. In this MI sensor, the magnetosensitive wire has at least a surface layer in which spins are aligned in a circumferential direction of the wire, and the high frequency electric current has a frequency in the range of 0.3 to 1.0 GHz inclusive.