To provide a dual tank which enables reduction of a manufacturing cost and omission of an operation for assembling tanks and facilitate an operation for fitting thereof, which dispenses with a strict weight control of a molding material, of which the wall thickness can be made uniform and which has an excellent outward appearance and dispenses with separate machining for holes, and the like.

A first divided body 20 and a second divided body 40 formed by dividing a first storage tank 1A, a second storage tank and a partition wall 5 vertically, laterally or longitudinally respectively are molded by an injection molding method, while a joining-side end face 30 in the first divided body 20 and a joining-side end face 50 in the second divided body 40 are joined together by a vibration welding method.