A straight-travelling/turning determination device for use in a motorcycle including: a front wheel rotational speed detector for detecting a rotational speed of a front wheel of a vehicle; a rear wheel rotational speed detector for detecting a rotational speed of a rear wheel of said vehicle; a rotational speed comparison value calculator for calculating a rotational speed comparison value (Vdiff), which is a value resulting from comparison between the rotational speed of the front wheel and the rotational speed of the rear wheel; and a straight-travelling/turning determiner that determines whether said vehicle is travelling straight or turning based on the magnitude of the rotational speed comparison value (Vdiff); the straight-travelling/turning determination device being characterized by including: a vehicle speed detector that detects vehicle speed; and an acceleration detector that detects vehicle acceleration; wherein the straight-travelling/turning determiner determines a straight-travelling rotational speed comparison value (Vdiff_s) as the rotational speed comparison value in a state where the vehicle speed is equal to or lower than a first predetermined speed and an absolute value of the vehicle acceleration is equal to or smaller than a predetermined acceleration and a blinker is not being actuated, and determines that the vehicle is turning if a ratio or difference between the rotational speed comparison value (Vdiff) and the straight-travelling rotational speed comparison value (Vdiff_s) becomes larger than a threshold value. a front wheel rotational speed detector for detecting a rotational speed of a front wheel of a vehicle; a rear wheel rotational speed detector for detecting a rotational speed of a rear wheel of said vehicle; a rotational speed comparison value calculator for calculating a rotational speed comparison value (Vdiff), which is a value resulting from comparison between the rotational speed of the front wheel and the rotational speed of the rear wheel; and a straight-travelling/turning determiner that determines whether said vehicle is travelling straight or turning based on the magnitude of the rotational speed comparison value (Vdiff); the straight-travelling/turning determination device being characterized by including: a vehicle speed detector that detects vehicle speed; and an acceleration detector that detects vehicle acceleration; wherein the straight-travelling/turning determiner determines a straight-travelling rotational speed comparison value (Vdiff_s) as the rotational speed comparison value in a state where the vehicle speed is equal to or lower than a first predetermined speed and an absolute value of the vehicle acceleration is equal to or smaller than a predetermined acceleration and a blinker is not being actuated, and determines that the vehicle is turning if a ratio or difference between the rotational speed comparison value (Vdiff) and the straight-travelling rotational speed comparison value (Vdiff_s) becomes larger than a threshold value. a vehicle speed detector that detects vehicle speed; and an acceleration detector that detects vehicle acceleration; wherein the straight-travelling/turning determiner determines a straight-travelling rotational speed comparison value (Vdiff_s) as the rotational speed comparison value in a state where the vehicle speed is equal to or lower than a first predetermined speed and an absolute value of the vehicle acceleration is equal to or smaller than a predetermined acceleration and a blinker is not being actuated, and determines that the vehicle is turning if a ratio or difference between the rotational speed comparison value (Vdiff) and the straight-travelling rotational speed comparison value (Vdiff_s) becomes larger than a threshold value.