A method was developed to detect the magnetic field signal and the laser frequency drift with the same photodetectors and preamplifiers in a ring laser system for magnetic field measurement. A digital steady-state error-free frequency stabilizer used in the system is described in detail. An accuracy of 2×10-5±1 Hz in the measurement of beat frequency arisen from a magnetic field and a laser frequency stability of 5×10-9 for an averaging time of 1 sec were obtained.