Modulation phase drift is observed in the interferometric fiber optic gyroscope (IFOG) and is attributed to the use of a multifunction integrated optical device. The influence of modulation phase drift on the IFOG's properties is analyzed in theory. The analysis results indicate that the drift causes a nonnegligible dead zone in the IFOG's response. To suppress the drift, three methods are proposed and validated experimentally. It is confirmed that a multifunction integrated optical device without a buffer layer can suppress the modulation phase drift effectively.