A traction control system in vehicle comprises a detector for detecting a monitored value which changes according to a degree of a drive wheel slip; a condition determiner for determining whether or not the monitored value meets a control start condition and whether or not the monitored value meets a control termination condition; and a controller for executing traction control to reduce a driving power of the drive wheel during a period of time from when the condition determiner determines that the monitored value meets the control start condition until the condition determiner determines that the monitored value meets the control termination condition; the condition determiner being configured to set at least the control start condition variably based on a slip determination factor which changes according to a vehicle state and such that the control start condition changes more greatly according to the vehicle state than the control termination condition.