A method of injection blow molding a synthetic resin bellows product having root portions and crest portions, each of which has the axially sectional form of a circular arc, and incline portions therebetween, for example, a plastic bellows product such as a constant-velocity joint boot, a steering boot or the like. The method is capable of molding a product having a thickness uniform throughout, and particularly, root portions having little dimensional variations. The method involves firstly molding the parison root portions or the root portions plus their adjacent areas into substantially the same shape as those of a final product by injection molding during molding of a parison, and then blow molding only the crest portions and the incline portions to obtain the shape of the final product.