A safety door having a rotary body hinged to a door frame, and a slider overlapping with an end of the rotary body and coupled thereto in such a way as to move leftwards and rightwards, the safety door comprising: a fastening portion provided on at least one surface of an overlapping region of the rotary body and the slider, thus preventing separation and deformation of the overlapping region and guiding a movement of the slider; and a stopper preventing the slider from being removed from the rotary body, and fixing or restricting a position of the slider, wherein the rotary body and the slider decrease a width of the safety door by an overlapping movement of a female guide groove and a male protrusion, characterized in that the safety door further comprises: a resilient pad provided between the protrusion and the guide groove, and supporting an impact generated by a reduction in the width of the safety door, wherein the fastening portion comprises: one or more fastening projections provided on either the protrusion or a cover of the guide groove at regular intervals in a vertical direction; and one or more fastening grooves provided on the other one of the protrusion and a cover of the guide groove, wherein the one or more fastening grooves are configured to slidably engage with the one or more fastening projections in a horizontal direction. a fastening portion provided on at least one surface of an overlapping region of the rotary body and the slider, thus preventing separation and deformation of the overlapping region and guiding a movement of the slider; and a stopper preventing the slider from being removed from the rotary body, and fixing or restricting a position of the slider, wherein the rotary body and the slider decrease a width of the safety door by an overlapping movement of a female guide groove and a male protrusion, characterized in that the safety door further comprises: a resilient pad provided between the protrusion and the guide groove, and supporting an impact generated by a reduction in the width of the safety door, wherein the fastening portion comprises: one or more fastening projections provided on either the protrusion or a cover of the guide groove at regular intervals in a vertical direction; and one or more fastening grooves provided on the other one of the protrusion and a cover of the guide groove, wherein the one or more fastening grooves are configured to slidably engage with the one or more fastening projections in a horizontal direction. a resilient pad provided between the protrusion and the guide groove, and supporting an impact generated by a reduction in the width of the safety door, wherein the fastening portion comprises: one or more fastening projections provided on either the protrusion or a cover of the guide groove at regular intervals in a vertical direction; and one or more fastening grooves provided on the other one of the protrusion and a cover of the guide groove, wherein the one or more fastening grooves are configured to slidably engage with the one or more fastening projections in a horizontal direction. one or more fastening projections provided on either the protrusion or a cover of the guide groove at regular intervals in a vertical direction; and one or more fastening grooves provided on the other one of the protrusion and a cover of the guide groove, wherein the one or more fastening grooves are configured to slidably engage with the one or more fastening projections in a horizontal direction.