A circuit unit accommodation box comprising a box body having at least one circuit unit accommodating part into which a circuit unit such as an ECU, which is equipped with a first connector, is inserted, and a receptacle electrical connector that is disposed in the circuit unit accommodating part and that makes an electrical connection with the inserted circuit unit, the receptacle electrical connector comprising: a cover part that receives the circuit unit and guides the circuit unit to a specified position, the cover part comprising locking projections that are provided on an upper wall and a lower wall of the cover part, the locking projections being configured to enter corresponding openings (13a, 13b, 14a, 14b) of the box body for locking the cover part to the box body; a second connector that is accommodated inside the cover part and locked with the cover part by locking means and that mates with the first connector and establishes electrical connection with the first connector when the received circuit unit is inserted into a first position, and unlocking means for unlocking the second connector from the locking means when the circuit unit is inserted from the first position into a position further toward the interior, with the second connector becoming slidable inside the cover part in the direction of insertion and withdrawal of the circuit unit after the second connector is unlocked by the unlocking means, and wherein the box body and the circuit unit comprise holding means for holding the circuit unit in the box body when the circuit unit is inserted from the first position to a second position via the interior position, the holding means comprising: first, second and third vertically extending ribs (63, 64, 65) provided on side surfaces of the circuit unit, the first and third ribs (63, 65) having inclined surfaces (63a, 65a) on respective front sides and the second rib having a perpendicular surface on the rear side; spring parts (15a, 15b) provided on the right and left sides of the circuit unit accommodating part, wherein the spring parts (15a, 15b) are configured to engage the inclined surfaces (63a, 65a) of the first and third ribs (63, 65) in order to drive the circuit unit in a direction opposite to the direction of insertion; a locking part on the wall of the circuit unit accommodating part, the locking part being configured to restrict the movement in the direction opposite to the direction of insertion by locking the perpendicular surface of the second rib in order to prevent wobbling. a cover part that receives the circuit unit and guides the circuit unit to a specified position, the cover part comprising locking projections that are provided on an upper wall and a lower wall of the cover part, the locking projections being configured to enter corresponding openings (13a, 13b, 14a, 14b) of the box body for locking the cover part to the box body; a second connector that is accommodated inside the cover part and locked with the cover part by locking means and that mates with the first connector and establishes electrical connection with the first connector when the received circuit unit is inserted into a first position, and unlocking means for unlocking the second connector from the locking means when the circuit unit is inserted from the first position into a position further toward the interior, with the second connector becoming slidable inside the cover part in the direction of insertion and withdrawal of the circuit unit after the second connector is unlocked by the unlocking means, and wherein first, second and third vertically extending ribs (63, 64, 65) provided on side surfaces of the circuit unit, the first and third ribs (63, 65) having inclined surfaces (63a, 65a) on respective front sides and the second rib having a perpendicular surface on the rear side; spring parts (15a, 15b) provided on the right and left sides of the circuit unit accommodating part, wherein the spring parts (15a, 15b) are configured to engage the inclined surfaces (63a, 65a) of the first and third ribs (63, 65) in order to drive the circuit unit in a direction opposite to the direction of insertion; a locking part on the wall of the circuit unit accommodating part, the locking part being configured to restrict the movement in the direction opposite to the direction of insertion by locking the perpendicular surface of the second rib in order to prevent wobbling.