A device, comprising: a flexible display (31; 34); a length measuring unit configured to measure a length of the flexible display expanded from the device; a speed measuring unit configured to measure a speed at which the flexible display expands from the device; and a processor configured to control the flexible display, the length measuring unit, and the speed measuring unit, wherein as the flexible display expands through a plurality of unit lengths, the processor is configured to display on the flexible display an application execution screen according to the length measured by the length measuring unit and to the speed measured by the speed measuring unit, wherein if the flexible display expands less than a first unit length (L1, 80) the processor is configured to display a first application execution screen corresponding to the first unit length or a second application execution screen corresponding to a second unit length on the flexible display, according to a first expansion speed of the flexible display, wherein if the first expansion speed is slower than a designated speed, the processor is configured to display the first application execution screen on the flexible display and if the first expansion speed is faster than the designated speed, the processor is configured to display the second application execution screen on the flexible display and wherein the first expansion speed is the speed of the flexible display measured at a beginning moment of the expansion, wherein if the flexible display expands more than the first unit length and less than the second unit length, the processor is configured to display the first application execution screen corresponding to the first unit length or the second application execution screen corresponding to the second unit length according to a second expansion speed of the flexible display, wherein if the first application execution screen is displayed on the flexible display, the processor is configured to display an enlarged first application execution screen on the flexible display when the second expansion speed is slower than the designated speed and the processor is configured to display the second application execution screen on the flexible display when the second expansion speed is faster than the designated speed, wherein the second expansion speed is the speed measured at the moment in which the flexible display is expanded as much as the first unit length; the second unit length being more than a length of the first unit length. a flexible display (31; 34); a length measuring unit configured to measure a length of the flexible display expanded from the device; a speed measuring unit configured to measure a speed at which the flexible display expands from the device; and a processor configured to control the flexible display, the length measuring unit, and the speed measuring unit, wherein as the flexible display expands through a plurality of unit lengths, the processor is configured to display on the flexible display an application execution screen according to the length measured by the length measuring unit and to the speed measured by the speed measuring unit, wherein if the flexible display expands less than a first unit length (L1, 80) the processor is configured to display a first application execution screen corresponding to the first unit length or a second application execution screen corresponding to a second unit length on the flexible display, according to a first expansion speed of the flexible display, wherein if the first expansion speed is slower than a designated speed, the processor is configured to display the first application execution screen on the flexible display and if the first expansion speed is faster than the designated speed, the processor is configured to display the second application execution screen on the flexible display and wherein the first expansion speed is the speed of the flexible display measured at a beginning moment of the expansion, wherein if the flexible display expands more than the first unit length and less than the second unit length, the processor is configured to display the first application execution screen corresponding to the first unit length or the second application execution screen corresponding to the second unit length according to a second expansion speed of the flexible display, wherein if the first application execution screen is displayed on the flexible display, the processor is configured to display an enlarged first application execution screen on the flexible display when the second expansion speed is slower than the designated speed and the processor is configured to display the second application execution screen on the flexible display when the second expansion speed is faster than the designated speed, wherein the second expansion speed is the speed measured at the moment in which the flexible display is expanded as much as the first unit length; the second unit length being more than a length of the first unit length.