A connector waterproofing structure watertightly seals a gap between openings of cavities of a pair of housings which accommodate terminals. In the structure, the pair of housings includes annular members formed at each of ends of the openings. The annular members are made of resin, protrude and surround the openings, either one of the annular members is formed into such a shape to be pressed to an inner peripheral surface or an outer peripheral surface of the opposite annular member, when the pair of housings are fitted together, and the inner peripheral surface or the outer peripheral surface are obliquely formed so that the wall thickness of the opposite annular member is gradually increased from the distal end toward the back.