A safe operating region in which the state of a complete mixing type reactor moves spontaneously towards a stable steady state without manipulative action is determined by the tracking function analysis. The tracking function, which is a linear combination of temperature and concentration of reactant, is displayed with a straight line on the phase plane and the line moves with time in the direction predicted by the derivative of the tracking function with respect to time. If straight line segments which correspond to tracking functions are assemble to form a closed polygon around a stable steady state on the phase plane and each segment moves towards the steady state as time passes, then the polygon manifests a boundary of safe operating region. Safe operating regions of the reactor in which dehydration and decomposition of methanol proceed simultaneously are shown with a tetragon and a heptagon around the stable steady states.