An external device for automatically opening/closing of a window or a door, consisting of a housing with a mounting means for fixed attachment to an opening sash or to a frame of a window/door, wherein in the housing are arranged: • a stepper motor with an electronic control unit comprising a driver and controller with a microprocessor adapted to supply control pulses for the direction, speed and rate of the motor's rotation, and with a power source connection means, • means for transforming the motor's rotational motion into a linear (9, 13), • an extended flexible connection (10, 11) that is rigid in its extension direction and is flexible in more than one direction transversely to its extension, said directions laying in different planes, the flexible connection being located in a leading groove in the housing with at least one curved section for changing direction of movement of the flexible connection, wherein one end of the flexible connection being connected to the motion-transforming means and the other end of the flexible connection passing freely through an opening in the housing and being connected to a support element that has means for attaching to window/door frame, wherein the motion-transforming means (9, 13) is adapted to move the flexible connection (10, 11) reciprocating between front-end and rear-end final positions so that when the axis of the motor is rotated in one direction the flexible connection is retracted into the housing to the rear-end final position and pulls the support element and when the axis of the motor is rotated in opposite direction, the flexible connection extends beyond the housing to the front-end final position and pushing the support element. • a stepper motor with an electronic control unit comprising a driver and controller with a microprocessor adapted to supply control pulses for the direction, speed and rate of the motor's rotation, and with a power source connection means, • means for transforming the motor's rotational motion into a linear (9, 13), • an extended flexible connection (10, 11) that is rigid in its extension direction and is flexible in more than one direction transversely to its extension, said directions laying in different planes, the flexible connection being located in a leading groove in the housing with at least one curved section for changing direction of movement of the flexible connection, wherein one end of the flexible connection being connected to the motion-transforming means and the other end of the flexible connection passing freely through an opening in the housing and being connected to a support element that has means for attaching to window/door frame, wherein the motion-transforming means (9, 13) is adapted to move the flexible connection (10, 11) reciprocating between front-end and rear-end final positions so that when the axis of the motor is rotated in one direction the flexible connection is retracted into the housing to the rear-end final position and pulls the support element and when the axis of the motor is rotated in opposite direction, the flexible connection extends beyond the housing to the front-end final position and pushing the support element.