In a boat attachment structure, an inner circumferential groove is formed at an open end of a cylinder bore of a caliper. A deep bottom section and a shallow bottom section are provided on the bottom of the inner circumferential groove, and therefore a step is formed between them. A dust boot having of a cylindrical elastic body with a thickened engagement section on the peripheral surface is inserted in the cylinder bore, and the engagement section is fit in the inner circumferential groove. Since each of chips produced during machining of the groove is parted by the step, chips will not remain in the inner circumferential groove.