A rough road determination device (100) is mounted on a vehicle (1) that is equipped with an internal combustion engine (10) having a crank angle sensor (31). This rough road determination device is equipped with determination means (20) for determining whether or not a road on which the vehicle travels is a rough road, by comparing a first amplitude fluctuation amount as an amplitude fluctuation amount of one pulse of an output of the crank angle sensor, which is determined in advance on the basis of a rotational speed of the internal combustion engine, with a second amplitude fluctuation amount as an amplitude fluctuation amount of one pulse that is actually output from the crank angle sensor.