Abstract: Described are apparatus and method for monitoring revolution speed values detected by corresponding revolution speed sensors too determine whether at least any one of an engine revolution speed sensor, a continuously variable transmission input axle revolution speed sensor, and a continuously variable transmission output axle revolution speed sensor has failed according to detected values of the engine revolution speed sensor, the input axle revolution speed sensor, and the output axle revolution speed sensor and a detected value of an instantaneous gear shift ratio when a lock-up mechanism is in a lock-up state. If all of three formulae on predetermined correlations are established, all of the sensors are determined to be normally operated. If any one or two of the three formulae are not established, the failure one of the three sensors is identified. The three correlation formulae are based as follows: 1) N.sub.E =N.sub.PRI ; 2) N.sub.PRI =N.sub.SEC .times.CN; and 3) N.sub.E =N.sub.PRI .times.C.sub.