Abstract: The invention relates to monitoring line sag of objects, especially monitoring the line sag of geographically wide objects, such as electric lines. The apparatus according to the invention can be fastened to the object to be monitored and it comprises a physical sensor, a data processing unit operationally connected to the sensor and communication means for relaying the data wirelessly to outside the apparatus from the data processing unit. According to the invention the sensor is arranged to recognize the movement or change of movement of the apparatus and the data processing unit is arranged to derive a parameter describing the state of the object on the basis of the movement data provided by the sensor. The invention also relates to a system and a method. The invention allows monitoring the status of electric lines inexpensively and in a centralized way.