An electrical connector assembly, comprising: a cable connector; a receptacle connector having a recessed portion for accommodating a substantially entire portion of the cable connector so that the receptacle connector is connected to the cable connector in a first direction which is a fitting direction corresponding to an up-and-down direction; a locking protruding portion (21; 21') disposed on one of the cable connector and the receptacle connector, said locking protruding portion (21; 21') including a protruding portion front edge (21 A; 21'A) and a protruding portion rear edge, said locking protruding portion (21; 21') having a length A in a second direction perpendicular to the first direction, the second direction corresponding to a front-back direction, and the length A being a length between a backmost position of the protruding portion rear edge and a forefront position of the protruding portion front edge (21A; 21'A) at the time when the locking protruding portion (21; 21') is in a horizontal attitude; and a locking groove portion (57; 57') disposed in the other of the cable connector and the receptacle connector, said locking groove portion (57; 57') having a groove front edge and a groove rear edge, said locking groove portion (57; 57') having a distance B in the second direction smaller than the length A, the distance B being a minimum width between a backmost position of the groove portion rear edge and a forefront position of the groove portion front edge at the time when the locking groove portion (57; 57') is in a horizontal attitude, so that the locking protruding portion (21; 21') can be inserted into the locking groove portion (57; 57') only in an inclined state in an upward attitude without taking the horizontal attitude, and the cable connector is prevented from coming off when the cable connector and the receptacle connector are in the horizontal state upon completion of regular fitting. a cable connector; a receptacle connector having a recessed portion for accommodating a substantially entire portion of the cable connector so that the receptacle connector is connected to the cable connector in a first direction which is a fitting direction corresponding to an up-and-down direction; a locking protruding portion (21; 21') disposed on one of the cable connector and the receptacle connector, said locking protruding portion (21; 21') including a protruding portion front edge (21 A; 21'A) and a protruding portion rear edge, said locking protruding portion (21; 21') having a length A in a second direction perpendicular to the first direction, the second direction corresponding to a front-back direction, and the length A being a length between a backmost position of the protruding portion rear edge and a forefront position of the protruding portion front edge (21A; 21'A) at the time when the locking protruding portion (21; 21') is in a horizontal attitude; and a locking groove portion (57; 57') disposed in the other of the cable connector and the receptacle connector, said locking groove portion (57; 57') having a groove front edge and a groove rear edge, said locking groove portion (57; 57') having a distance B in the second direction smaller than the length A, the distance B being a minimum width between a backmost position of the groove portion rear edge and a forefront position of the groove portion front edge at the time when the locking groove portion (57; 57') is in a horizontal attitude, so that the locking protruding portion (21; 21') can be inserted into the locking groove portion (57; 57') only in an inclined state in an upward attitude without taking the horizontal attitude, and the cable connector is prevented from coming off when the cable connector and the receptacle connector are in the horizontal state upon completion of regular fitting.