Mass-controlled layer-by-layer sequential adsorption process was newly developed for the fabrication of ultra-thin organic films formed by polyelectrolytes. A quartz crystal was attached to the arm of a robot and, using the quartz crystal microbalance (QCM) method, the frequency shifts during adsorption of the materials were monitored. By optimization of the feedback constant of the. data acquired by QCM to the dipping time, high-quality self-assembled films were produced. From the cross-sectional images observed by TEM and thickness measurement carried out using an ellipsometer, it was found that the layer thickness was controlled with nm-scale accuracy.