A simple method for the measurement of ion drift velocity in mother gas has been developed. In this method, ion density is modulated in time in sinusoidal forms and the phase delay of the fundamental component of the modulated ion current is measured with a lock-in amplifier in place of DC ion current measurement by the two-shutter method. The phase delay of the fundamental component in differential forms gives accurate ion transit times through the drift distance. The mobility data of Ar- in Ar obtained using this method are in good agreement with those obtained by the two-shutter method. Although it involves simple procedures, this method will be useful for accurate measurement of the drift velocity of ions over a wide range of reduced electric fields because it can exclude the effects of ion diffusion and the change of ion species.