The present invention is a production method for an expansion balloon (2) for use in a balloon catheter (1), including the steps of; forming a balloon tube (8) by extrusion molding so as to have a cross section orthogonal to an axial direction with the shape of a circle on the outside and the shape of a polygon having a circumcircle on the inside; and placing the balloon tube (8) in a mold to perform biaxially-stretched blow molding to obtain the balloon, whereby it is possible to produce the expansion balloon (2) that has no unevenness in film thickness of a straight pipe part (4) and allows stable folding control by simple processes with favorable molding yields.