A failure determination system for a vehicle speed detection device, which, in a case where only one vehicle speed detection device is used, can always accurately perform failure determination of the device, and ensure a high marketability, while avoiding increases in manufacturing costs and product prices of the system. An ECU calculates an engine speed, calculates a detected vehicle speed based on a detection signal from a vehicle speed sensor, calculates an estimated total transmission gear ratio indicative of correlation between the detected vehicle speed and the engine speed, stores the estimated total transmission gear ratio, calculates a reference vehicle speed using a value of the estimated total transmission gear ratio, calculated predetermined times earlier, and the engine speed, and determines a failure of the sensor using a for-use-in-monitoring vehicle speed error which indicates an error between the reference vehicle speed and the detected vehicle speed.