To provide a waterproof interconnecting structure of a metallic roof plate capable of omitting a necessary sealing compound by a conventional technique as a constitution for maintaining indoor pressure of a back pressure room so that it is made to combine with the prevention of a female-shaped protrusive extraction by maintaining structure of the indoor pressure of the back pressure chamber.

An upper projection bending outer fitting engaging section 30 of a down side wall section 24 of a female-shaped protrusion 23 and a lower projection bending outer fitting engaging section 28 are made to respectively adherently abutted on an upper projection bending inner fitting engaging section 10 of a rising side wall section 4 of a male-shaped protrusion 3 and a lower projection bending inner fitting engaging section 8 to make them fit by elastic restoring force of the female-shaped protrusion 23. The back pressure room 11 for stormwater invasion prevention is formed between an outside recess being section 29 and an inside recess bending section 9. The chamber lower end section 11a of the back pressure chamber 11 is closed by adherently abutted on a lower projection bending outer fitting engaging section 28 of a descending side wall section 24 of the female-shaped protrusion 23 and a lower projection bending inner fitting enagaging section 8 of the rising side wall section 4 of the male-shaped protrusion 3. The chamber upper end section 11b of the back pressure chamber 11 is closed by adherently abutted on an upper projection bending outer firring engaging section 30 of the down side wall section 24 of the female-shaped protrusion 23 and the upper projection bending inner fitting engaging section 10 of the rising side wall section 4 of the male-shaped protrusion 3.