A quantitative measurement method for two dimensional thickness distribution of molecularly thin lubricant films on magnetic disks is presented. Visualization of a molecularly thin film is an extremely useful technique for the investigation of replenishment and retention of thin lubricant films. We demonstrated ellipsometric microscopy using a white light source could provide improvement of signal to noise ratio of images in our previous works. However, measurement of absolute thickness of films has not been established, although the relative thickness can be obtained by intensity of images. In this report, method for measuring the absolute thickness of films has been presented. In this method, by using the relationship between the polarizer angle and the film thickness, the thickness measurement with a sub nm accuracy has been demonstrated.