A straight-traveling/turning determination device includes a straight-traveling/turning determiner that determines whether a motorcycle is traveling straight or turning by comparison between a threshold and a rotary speed comparison value that is a value resulting from comparison between the rotary speed of a front wheel and the rotary speed of a rear wheel. The straight-traveling/turning determiner uses the rotary speed comparison value in the state in which the vehicle speed is equal to or lower than first predetermined speed and an absolute value of the vehicle acceleration is equal to or lower than predetermined acceleration and a turn signal is not being actuated as a straight-traveling rotary speed comparison value, and determines that the motorcycle is turning if a ratio or difference between the rotary speed comparison value and the straight-traveling rotary speed comparison value becomes larger than a threshold.