The purpose of this invention is to provide a balloon catheter whereby rupture of the balloon due to liquid being forced into the balloon with the balloon extended or treatment with a balloon shape that is unsuitable for treatment can be prevented. This invention provides a balloon catheter that comprises the following: an inside-tube shaft that has a narrow inside-tube section at the distal end and a wide inside-tube section at the proximal end; an outside-tube shaft with the inside-tube shaft inserted into the lumen thereof; a balloon, the distal end of which is affixed to the distal end of the inside-tube shaft and the proximal end of which is affixed to the distal end of the outside-tube shaft, that changes between a normal shape and an extended shape as a result of the outside-tube shaft and the inside-tube shaft sliding against each other; and a housing that connects to the lumen of the outside-tube shaft and has a narrow lumen at the distal end and a wide lumen at the proximal end. When the balloon is extended, the wide inside-tube section of the inside-tube shaft is inserted into the narrow lumen of the housing, blocking the insertion of liquid into the balloon.