An auxiliary mechanism for a seat belt apparatus includes a support portion and a guide portion. The support portion supports a webbing in a manner drawable to the front of a seat back. The support portion is movable between a storage position, which is near the seat back, and a projection position, which is spaced further apart from the seat back than the storage position to draw out the webbing toward the front of the seat back. The guide portion guides the movement of the support portion between the storage position and the projection position, and then continuously guides the movement of the support portion along a vertical direction of the seat back at the storage position.