A balloon dilator having a positioning part whose form is changed so as to be fit to a channel of an endoscope; and a larger-diameter part, which is connected to the positioning part and is inflated to have a diameter larger than that of the positioning part. Preferably, the positioning part is arranged on a side of the larger-diameter part, which is closer to an operator's hand, so that the positioning part communicates with and can be inflated together with the larger-diameter part. When the form of the positioning part is changed, the positioning part has a substantially constant outer-diameter along the axial direction thereof, or the outer diameter of the positioning part gradually decreases from the side connected to the larger-diameter part, to the other base end side.