To provide a waterproofing method obtaining high waterproofing effect without taking time and effort.

In the waterproofing method a waterproofing sheet material 6 covers an area ranging from an annular groove 3 formed over a whole periphery of a shaft part 1A on the top end side to a waterproofing layer 5 of a roof part and a waterproofing filling material 8 is filled into the annular groove 3 in a state that a top end of the waterproofing sheet material 6 is positioned in the annular groove 3 and a base part of the waterproofing sheet material 6 is adhered on the waterproofing layer 5 with respect to a protruding mounting fixture having a shaft part 1A protrudingly erected on the roof part with a mounting part 1B at the top end of the shaft part 1A. The waterproofing sheet material 6 is formed into an externally fittable cylindrical shape over a whole length or almost whole length of the shaft part 1A and a collar-shaped part 6B along the waterproofing layer 5 is formed at its base part, and a C ring shaped plate 7 for adhereing the waterproofing sheet is fitted in the annular groove 3, and the cylindrical waterproofing sheet material 6 is externally fitted to the shaft part 1A and the top end edge part of the waterproofing sheet material 6 is adhered on the plate 7 with the top end edge folded into the annular groove 3.