To provide a method of forming a waterproofing film in a building, according to which a urethane waterproofing film having water vapor permeability is efficiently formed over the entire waterproofing objective surface having a large area, while adjusting the film so as to have a positively uniform thickness of 10 mm or more.

According to the method, a liquid mixture 22 containing a base compound 3 and a hardener 4 thereof which are mixed beforehand in an ejecting machine 21, is ejected from the ejecting machine 21 along a current of compressed air to be sprayed to the waterproofing objective surface 23, to thereby form a waterproofing film substrate layer 51 having a predetermined thickness D. Then a plurality of mat-like film thickness setting members 52 each having vapor permeability and a predetermined thickness D2 are placed on the waterproofing objective surface 23, and the liquid mixture 22 is sprayed to each gap between the two film thickness setting members 52 adjacent to each other, until the accumulated height becomes almost the same as the thickness D2 of the film thickness setting member 52, and by hardening the liquid mixture 22 accumulated until the film thickness setting member 52 is embedded therein, the urethane waterproofing film 57 integrally united with the waterproofing film substrate layer 51 is formed.