To facilitate mounting and demounting work of a float on and from a jack cylinder.

This float mounting device of a construction machine to mount an upper end part of the float on a lower end part of the jack cylinder 14 to jack up a frame free to connect and disconnect is furnished with a lever member 16 mounted on a cylinder receiver part 153 provided on the upper end part of the float and a supporting means 17 to support the lever member 16 free to move and holding mechanisms 16a, 16b to hold the lever member 16 at a retreating position at an engaging position to be engaged with a groove part 142 formed on a lower part outer peripheral surface of the jack cylinder 14 and the retreating position to retreat to the outer diametrical side from the lower part outer peripheral surface.