Tension detecting device

An end portion of a webbing belt is attached to a metal base plate, in which a hole is formed. A projecting portion is formed at an inner edge of the hole which inner edge is at a side opposite a belt attachment portion. When tension is applied to the webbing belt, due to a fixed plate engaging with the projecting portion, a tensed portion of the metal base plate between the belt attachment portion and the hole extends and the projecting portion contracts in the direction of the tension. As a result, first strain gauges mounted to the tensed portion output a signal corresponding to extension, and second strain gauges mounted to the projecting portion output a signal corresponding to contraction. A tension detecting device detects the tension on the basis of a large difference in outputs of the first strain gauges and the second strain gauges.