A dilation-assisting member is easily disposed at a gap between a balloon and a site to be dilated and does not drop off even if the balloon is expanded extremely exceeding a predetermined diameter.In a balloon assembly for a catheter 1 having, on an external surface of a balloon 3 the dilation-assisting member 4 which dilates a stenosis site the dilation-assisting member is extended from a proximal end portion to a distal end portion of the balloon and one end of the dilation-assisting member is fixed to a first support member 5 provided at the proximal end portion and the other end of the dilation-assisting member is held slidably by a second support member 6 provided at the distal end portion, and the other end of the dilation-assisting member is protruded from the second support member such that the dilation-assisting member 4 has a length which enables the dilation-assisting member to extend from the proximal end portion to the distal end portion even when the balloon 3 is expanded.
