A connector includes a housing and a front holder. The front holder includes an arm portion to be exposed at a side surface of the housing. The housing includes a pair of extension walls on the side surface of the housing to cover the arm portion. The pair of extension walls has a first part and a second part. In the first part, a distance between the extension ends of the extension walls is constant. In the second part, the distance increases gradually as they extend away from the first part in the attaching direction. A distal end of the arm portion is in a position corresponding to the second part when the front holder is in a final lock position, and the distal end of the arm portion is in a position corresponding to the first part when the front holder is in a preliminary lock position.