An optical fiber humidity sensor based on a thermal lens detection technique is proposed. This sensor allows us to measure humidity without special treatment of the clad of optical fiber in spite of a certain optical absorption path-length under 50µm. Because of this, we believe that our humidity sensor will have a long lifetime satisfying uses with severe requirements. Our sensor is also extremely small (e.g., 20mm in length and 5mm in diameter) and robust. It is also confirmed that our optical fiber humidity sensor can measure absolute humidity.