A straight running state is accurately determined using only wheel speed data in any running state. Based upon right and left wheel speed values calculated by a wheel speed calculating mechanism (13) from detection values of right and left wheel speed sensors (11, 12), a wheel speed ratio calculating mechanism (21), a distance calculating mechanism (31), and a distance ratio calculating mechanism (32) respectively calculate a right and left wheel speed ratio, an actual running distance, a linear distance, and a distance ratio of the two distances at every preset checkpoint (CP). A wheel speed ratio change amount evaluating mechanism (24) extracts CPs at times where a change amount of the wheel speed ratio is relatively small as effective CPs, and a determining mechanism (43) determines an effective CP with the smallest distance ratio from among the effective CPs as in a straight running state.