A cruise control system is adapted to a vehicle equipped with an internal combustion engine and a manual transmission which are disengageably connected through a clutch. The cruise control system is arranged to calculate a first vehicle operation indicative quantity based on an engine speed detected by an engine speed sensor, to calculate a second vehicle operation indicative quantity based on a wheel speed detected by a wheel speed sensor, to determine whether a deviation between the first vehicle operation indicative quantity and the second vehicle operation indicative quantity is greater than a predetermined value, and to cancel the cruise control when the deviation is greater than the predetermined value.