A flexible boot (1) having a bellows and mount portions (2, 3) on both axial ends of the bellows portion (4) at which the boot (1) is mounted on parts of a mechanism such as an automotive constant velocity joint (10). The flexible boot (1) has an anti-degradation agent make-up element (7, 7a, 7b) made of a material which contains an anti-degradation agent and which is capable of allowing the anti-degradation agent to precipitate on the surface thereof. The anti-degradation agent make-up element is fitted on at least a portion of the outer surface of the bellows (4) so that the anti-degradation agent precipitated on the surface of the material of the anti-degradation agent make-up element is caused to progressively immigrate into the material of the bellows (4) thereby making-up the anti-degradation agent in the bellows (4).
