PROBLEM TO BE SOLVED: To provide a facility installing foundation of a deck backing roof and a construction method thereof, capable of inexpensively installing this facility, even if an interval of installation means provided in the facility is different from an interval of a crest surface of a steel deck.SOLUTION: The facility installing foundation comprises a lower member 20 fixed by being extended to a crest surface 13a and a crest surface 13b of the steel deck 10, an upper member 40 fixed to the lower member 20 via a lower heat insulation material 30 and provided with facility installing means 45 for installing the facility 500 on an upper surface, a roof heat insulation material 91 for covering the steel deck 10 in a range except for the lower heat insulation material 30, an upper heat insulation material 50 for blocking up between the roof heat insulation material 91 and the upper member 40 and a waterproof reinforcement sheet 60 for covering the roof heat insulation material 91 and the upper heat insul
