We propose an optical heterodyne method to measure the magnetostatic field using a magnetic fluid with high sensitivity. This measurement system consists of a stabilized transverse Zeeman laser, an optical isolator with a Faraday rotator and a half-wave plate, and a sensor. The probe light passes through the magnetic fluid twice in the sensing part. These optical components are connected by the polarization-maintaining fiber. This fiber sensor is independent of temperature and external stress because both the reference and probe beat signals are compared for their phase difference using two bundle fibers. The magnitude of the magnetostatic field as well as its azimuthal direction is obtained from the phase difference. The results obtained are a resolution of ~0.1 Oe and a measurement of up to 500 Oe.