A method of controlling film thickness of a mixture liquid layer of oil material and water in a printer such as, for example, an offset printer is disclosed wherein the mixture liquid layer of the oil material containing printing ink, printer's varnish and the like and water such as, for example, dampening water attached to one roller of a roller group carrying the mixture liquid layer is alternately irradiated by infrared rays which are most strongly absorbed into the oil material and the water and by infrared rays which are hardly absorbed into the oil material and the water. Film thicknesses of the oil material and the water are detected on the basis of infrared absorption characteristics of the oil material and the water. The detected film thicknesses of the oil material and the water are compared with the respective predetermined target values and supply of the oil material and the water is controlled so that differences between the detected film thicknesses and the respective target values are minimized. Thus. printing failure such as so-called greasing and water stain occurring in a prior art printing is prevented and printing quality is improved, so that spoilages can be reduced.