To reduce cost required for monitoring forces in ground anchors, an innovative method is proposed. In this method, a specially designed mechanical switch is set on an anchor head. As the force changes, its tendon changes its length, which then is transferred to the shift of light blocking metal shield panel. When the shield panel moves, the number of plastic optical fiber receiving sun light changes, from which an approximate values of the anchor force can be identified. Once this sensor is set, a long term observation of anchor forces (though it is limited to observation during day time) becomes possible without using electricity.