Roughness sensors which operate by means of optical stylus have advantages of noncontact measurement, relatively long working distance, high re solution and high accuracy. However, they also have some weak points. The range of measurement is insufficient and the allowable amount of local inclination of the surface is not large enough. It is established that the error due to the inclination can be compensated better by the null method than by the usual method. In the present paper, a new method to realize the null method by oscillating the objective lens and detecting the zero-crossing point of the periodically changing focal error signal is presented. The trial sensor has accuracy of about 20 nm and a measuring range of 20μm. By measurillg the time interval of zero-crossing points of focal error signal as an alternative to its voltage, the possible dynamic range of the sensor is expected to be enlarged by more than 80 dB.