A connector housing with a mat seal, comprising: a connector housing, made of hard resin, and including a tubular-like hood whose front end is opened, and a rear wall which blocks a back opening of the hood and in which a plurality of terminal insertion holes into which terminals are respectively inserted from a back side are provided; and a mat seal provided with a thick, generally rectangular plate-shaped seal body, integrally formed with a front surface of the rear wall, by arranging a front mold and a back mold so that the rear wall is sandwiched inside and outside the hood of the connector housing, and injecting silicone gum forming material from a gate which is provided at the back mold into a seal forming cavity which is defined between the rear wall and the front mold, wherein through holes which respectively communicate with the terminal insertion holes of the rear wall of the connector housing are formed in the mat seal, wherein annular seal lips, which are tightly fitted to outer circumferences of electric wires which extend backwards from the terminals which are inserted inside the connector housing by passing through the terminal insertion holes and the through holes, are formed on internal circumferences of the through holes respectively, wherein, at an outer surface of the rear wall of the connector housing, a plurality of passages (15A-15D) for the silicone gum forming material, which are separated from each other, are provided outside an area where the plurality of terminal insertion holes are formed, at four corners of the generally rectangular plate-shaped seal body, wherein, on the outer surface of the rear wall of the connector housing and outside the passages for the silicone gum forming material, an annular rib is protruded from the outer surface and surrounds an area inside the annular rib, and wherein the mat seal is molded in the area inside the annular rib as a reservoir space in which the silicone gum forming material which overflows from the seal forming cavity is retained, and among the plurality of passages for the silicone gum forming material, using at least one passage as a passage for a gate through which the silicone gum forming material is injected into the seal forming cavity, and the other passages as passages for overflow. a connector housing, made of hard resin, and including a tubular-like hood whose front end is opened, and a rear wall which blocks a back opening of the hood and in which a plurality of terminal insertion holes into which terminals are respectively inserted from a back side are provided; and a mat seal provided with a thick, generally rectangular plate-shaped seal body, integrally formed with a front surface of the rear wall, by arranging a front mold and a back mold so that the rear wall is sandwiched inside and outside the hood of the connector housing, and injecting silicone gum forming material from a gate which is provided at the back mold into a seal forming cavity which is defined between the rear wall and the front mold, wherein through holes which respectively communicate with the terminal insertion holes of the rear wall of the connector housing are formed in the mat seal, wherein annular seal lips, which are tightly fitted to outer circumferences of electric wires which extend backwards from the terminals which are inserted inside the connector housing by passing through the terminal insertion holes and the through holes, are formed on internal circumferences of the through holes respectively, wherein, at an outer surface of the rear wall of the connector housing, a plurality of passages (15A-15D) for the silicone gum forming material, which are separated from each other, are provided outside an area where the plurality of terminal insertion holes are formed, at four corners of the generally rectangular plate-shaped seal body, wherein, on the outer surface of the rear wall of the connector housing and outside the passages for the silicone gum forming material, an annular rib is protruded from the outer surface and surrounds an area inside the annular rib, and wherein the mat seal is molded in the area inside the annular rib as a reservoir space in which the silicone gum forming material which overflows from the seal forming cavity is retained, and among the plurality of passages for the silicone gum forming material, using at least one passage as a passage for a gate through which the silicone gum forming material is injected into the seal forming cavity, and the other passages as passages for overflow.