1. A process control apparatus for an N- (where N is any positive-integer not less than 2) input/output process comprising: Proportional-Integral-Derivative (PID) controller means for controlling said process in accordance with PID parameters and control errors as differences between N outputs from said process and set-point signals for the N outputs; signal generating means for superposing N persistently exciting identification signals on the N inputs to said process; identifying means for identifying Z-transfer functions corresponding to all input-to-output combinations, in accordance with the N inputs to said process and the N outputs from said process, while the N persistently exciting identification signals are superposed on the N inputs; calculating means for calculating an S-transfer function of said process in accordance with identified Z-transfer functions; and tuning means for matching an S-transfer function between a set-point signal for a process control system and a process output which is obtained from a calculated S-transfer function with an S-transfer function of an N-input/output decoupled reference model with no interference between process variables, and for tuning the PID parameters of said PID controller means in accordance with matched results.