A direct visual monitoring system for sensing the interior of a rotary mineral mill which comprises a monitoring unit, a main control unit and an operation and management unit wherein the monitoring unit is located inside a feed hopper and adjusted according to the physical characteristics of said feed hopper and in accordance with the dimensions of the mill, so as to allow for a direct view of the interior of the mill. The operating method comprises arranging inside a monitoring unit, a container of sensors for sensing the inner temperature thereof; determining the acceleration on the vertical axis, on the horizontal lateral axis and on the front horizontal axis, according to the time; obtaining two-dimensional images of the geometric conditions of the interior of the mill; obtaining two-dimensional thermal images of the interior of the mill and executing a detection of distance in one or more planes.