1. A poor visibility estimation system that estimates poor visibility, comprising: a detecting sensor configured to detect at least one of a steering angle, a steering angular velocity, and a steering torque of a vehicle; and a microcomputer configured to perform as a visibility estimating section configured to estimate a poor visibility point or area based on at least one of the detected steering angle, steering angular velocity, and steering torque, wherein the detecting sensor is configured to detect the steering angle of the vehicle, the microcomputer is further configured to perform as a frequency converting section configured to subject data about the steering angle to frequency conversion, and the visibility estimating section is configured such that, if the spectrum area of the data subjected to the frequency conversion in a predetermined frequency band exceeds a spectrum area prescribed as a reference, the visibility estimating section determines that the visibility is poor at the point where the data is acquired or that the visibility is poor in an area including this point.