EXTENSION UNIT

An extension unit includes a buckle member, a first coupling member, a second coupling member, and a webbing belt. The buckle member is engaged with a first tongue plate of a seat belt apparatus, includes a first holder holding the webbing belt, and a fixing member fixing an end of the webbing belt. The second coupling member is disposed between the buckle member and the first coupling member and coupled to the first coupling member. The webbing belt joins the buckle member and the first coupling member. The second coupling member is so disposed that a projected region extending inward in a vehicle width direction is defined. The projected region has a size corresponding to a thickness of an operator's hand and is not overlapped with the buckle member. The second coupling member includes a second holder holding the webbing belt extending from the first holder to the fixing member.