The invention relates to a moisture sensor (100) for detecting relative atmospheric moisture. The inventive sensor comprises a moisture-sensitive element which undergoes a size change under the influence of atmospheric moisture. Said size change can cause bending of a section pertaining to an optical fibre (1). A change of the light flux through the optical fibre is a result of the extend of the bending, whereby said change is present as a measurable reference quantity in the form of measurable subduing of light of the optical fibre. The moisture-sensitive element is a so-called small hygro band (2) that moves a rounded head (10, 10'. 10'') over a gearbox. Said head presses against the section of the optical fibre (1) and thus causes subduing of light of the optical fibre, whereby said section has to be bend and pertains to the optical fibre.