Parallel turning attracts attention as one of the important technologies for the multi-tasking machine tools. This is because there is a potential to enhance the stability limits compared to turning operation using single tool when cutting conditions are properly selected. Although stability prediction models for parallel turning have been developed in recent years, in-process monitoring technique of chatter is almost out of focus. In this study, to suppress chatter vibration, unequal pitch turning method was proposed. In this method, the upper tool was controlled based on optimum pitch angle calculated from spindle speed and chatter frequency. Chatter frequency was identified from estimated cutting force by disturbance observer. From the result of parallel turning test, it is clear that chatter vibration can be suppressed by controlling the upper tool based on optimum pitch angle.