A flexible boot for universal joints including a joint body having an inner ring fixed to one end of a first shaft and an outer ring provided on one end of a second shaft comprises both ends fixed to the outer ring and to the first shaft, respectively, and a bellows portion positioned between the both ends and having at least one valley. There is provided at least one elastic holder ring closely contacted with an outer peripheral surface of the at least one valley to prevent acute folding of the valley of the flexible boot.