A male tab terminal has an O-ring mounting portion formed between its wire connection portion and its electrical contact portion, and this O-ring mounting portion is surrounded by an opening peripheral surface of an O-ring receiving step portion of a female connector housing which defines an opening of a male terminal receiving chamber. An O-ring, mounted on the O-ring mounting portion of the male tab terminal, is held in intimate contact with an outer peripheral surface of the O-ring mounting portion over an entire periphery thereof, and also is held in intimate contact with the opening peripheral surface of the O-ring receiving step portion over an entire periphery thereof. When a female connector and a male connector are fitted together, the O-ring is directly clamped between the female connector housing and a male connector housing in a direction of fitting of the female and male connectors, and is pressed by the two connector housings.