NOVELTY - The rolling element (30) has a sensor unit which is provided for sensing the slippage of the rolling element during operating process in which the rolling element moves. The sensing unit comprises at primary acceleration sensor and secondary acceleration sensor. The primary acceleration sensor is provided to sense acceleration of the rolling element vertically to a main symmetry axis of the rolling element. The sensing unit is provided to measure position of the center of gravity of the rolling element in inertial system.    USE - Rolling element for roller bearing used in wind power plant. Can also be used in transport apparatus in mining industry or storage facilities of ships or in large gear boxes.    ADVANTAGE - The rolling element with high efficiency can be obtained. The slippage of the rolling element, in particular the movement of rolling element relative to a bearing ring of the roller bearing differs from unreeling of the rolling element on the bearing ring, so that sliding of the rolling element on the bearing ring takes place. The sensing of slippage takes place qualitatively and quantitatively so that high efficiency is achieved. The compact and inexpensive movement monitoring can be achieved, and an optimum maintenance time dot can be found. The sensor measures the accelerations by the fact that inertia force affecting the test mass of sensor is determined. Thus simple construction of sensor can be achieved, and the sensing unit can be free from gyroscopes. The precise sensing of the slippage can be achieved. The running surface is imaged by rotation around any angle along the axles so that effective sensing can be achieved. The main symmetry axis intersects the test mass so that simple sensing and simple construction of output signal of the primary acceleration sensor can be achieved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method for sensing slippage of the rolling element.    DESCRIPTION OF DRAWING(S) - The drawings show the schematic view of the wind power plant and roller bearing with rolling element.    Roller bearing (26)    Rolling element (30)    Running surface (32)    Inner ring (44)    Outer ring (46)