A leg unit an outer tube with a fixing rod having several rod sections, each has two interconnected curved and straight sides, two curved sides along an axis defining a fixing channel; an inner tube inserted into the outer tube; and a carrier seat fixed to the inner tube, and a limiting seat seated within the carrier set such that the carrier and limiting seats can rotate together, the carrier seat has an axial hole, two stems extending transversely through the hole to restrict one rod section therebetween, the limit seat has a slide hole complementing with and permitting extension of the fixing rod; the inner tube is rotatable relative to the outer tube between an unlocked position, where movement between the inner and outer tubes is permitted and a locked position, where the stems are confined within the channels, thereby preventing movement between the inner and outer tubes.