In this study, a modal analysis of a high-speed processing machine capable of simultaneously machining the plane and side surfaces is performed to obtain the natural frequency. Moreover, the safety of the high-speed processing machine is described. The high-speed processing machine has four spindles: two spindles machine the plane, and the other two spindles machine the right and left sides of the workpiece. A modal analysis was performed on this high-speed processing machine to derive the results for the eight modes. Among them, the natural frequency of mode 1 (38.7 Hz), which was most suitable for this system, was selected. When the high-speed processing machine was fed at a speed of 40 m/min, the result of measuring the frequency used was 15.6 Hz, which is 2.47 times less than that of the modal analysis result. Therefore, a high-speed processing machine can be safely used by avoiding resonance.