A poor road determination device (100) is mounted in a vehicle (1) provided with an internal combustion engine (10) comprising a crank angle sensor (31). The poor road determination device is provided with a determination means (20) which compares a first amplitude fluctuation amount constituting the amplitude fluctuation amount of one output pulse of the crank angle sensor which is predetermined in accordance with the rotation speed of the internal combustion engine, and a second amplitude fluctuation amount constituting the amplitude fluctuation amount of one pulse which is actually output from the crank angle sensor, and thereby determines whether or not the road on which the vehicle is travelling is a poor road.