A seat belt tension adjuster adjusts tension on a seat belt wound on a reel by a biasing spring. The tension adjuster includes a biasing force adjusting mechanism situated between the reel and the spring for adjusting a biasing force transmitted from the spring to the reel depending on a rotational angular position of the reel. The biasing force adjusting mechanism includes a cam for gradually changing the biasing force based on the rotational angular position of the reel. The tension adjuster can freely set the withdrawing force and the winding force of the seat belt.