Abstract: Provision of a connector assembly which enables a half-fitted state to be reliably prevented and further enables a fitting force to be reduced. A connector assembly having a first housing in which an end of at least one first signal line is attached, a second housing in which an end of at least one second line to be connected to the end of the first signal line is attached and which fits with a fitting opening of the first housing, and a plate shaped fitting detection pin which is inserted from a detection insertion slot formed at a side of the first housing so as to pass through an approximate center axis of the first housing. The second housing is formed with a detection pin recess which allows the insertion of the fitting detection pin in a state where the second housing is fitted in the fitting opening of the first housing. The facing inner walls of the second housing constituting the detection pin recess are formed with projecting guide ribs.