A connector assembly (1,2) comprising: a first connector comprising a first housing comprising an outer housing, a mating section including an inner housing and a front housing the mating section forming an approximately rectangular shape having two long sides and two short sides when projected in a mating direction (z); a second connector comprising a second housing configured to mate with the first housing via inner housing, the second housing comprising grooves;the inner housing of the first housing being configured to move between a mating starting state (A1, B1) and a complete mating state (A3, B3) via a halfway mating state thereby to mate with the second housing; and spring members fixed to the first housing (30,70,100) and being configured to be inserted into the grooves provided in the second housing being mated therewith in a mating direction to be deformed elastically in a direction intersecting the mating direction (z) at the complete mating state (A3, B3); characterized in that the spring members are press-fitted into the two long sides and the two short sides of the inner housing of the rectangular mating section, the spring members are exposed from the inner housing and protrude therefrom in a mating direction (z) whereby the spring members are configured such that when the connector is in a mating starting state (A1, B1) or a half-way mating state the spring members are maintained in a state before elastic deformation and when the connector is in a complete mating state the spring members are pushed by the wall of the groove of the second housing to elastically deform at the time of mating thereby to prevent looseness between the housings and to reduce the mating force. a first connector comprising a first housing comprising an outer housing, a mating section including an inner housing and a front housing the mating section forming an approximately rectangular shape having two long sides and two short sides when projected in a mating direction (z); a second connector comprising a second housing configured to mate with the first housing via inner housing, the second housing comprising grooves;the inner housing of the first housing being configured to move between a mating starting state (A1, B1) and a complete mating state (A3, B3) via a halfway mating state thereby to mate with the second housing; and spring members fixed to the first housing (30,70,100) and being configured to be inserted into the grooves provided in the second housing being mated therewith in a mating direction to be deformed elastically in a direction intersecting the mating direction (z) at the complete mating state (A3, B3); characterized in that the spring members are press-fitted into the two long sides and the two short sides of the inner housing of the rectangular mating section, the spring members are exposed from the inner housing and protrude therefrom in a mating direction (z) whereby the spring members are configured such that when the connector is in a mating starting state (A1, B1) or a half-way mating state the spring members are maintained in a state before elastic deformation and when the connector is in a complete mating state the spring members are pushed by the wall of the groove of the second housing to elastically deform at the time of mating thereby to prevent looseness between the housings and to reduce the mating force.