A portable device comprising: a main display; a flexible display; a housing unit configured to contain the flexible display; and a measurement unit configured to measure a first length of the flexible display, wherein the first length is a length of the flexible display that is extended from the portable device; wherein the portable device further comprises a control unit configured to: obtain a second length of the flexible display, wherein the second length is a length of the flexible display that is not extended from the portable device using the first length when the flexible display is extended from the portable device, compare the second length with a width of the main display, display an indicator indicating the second length only on the main display according to the comparison result, and move the displayed indicator according to a change of the second length in a direction in which the flexible display is extended or reduced when the second length is equal to or less than the width of the main display, and control the displayed indicator not to be displayed on the main display while extension or reduction of the flexible display is stopped, wherein, when at least one application execution screen is displayed on the flexible display, the second length is obtained by subtracting the first length from a total width of the application execution screen, and wherein the indicator is displayed as a virtual contour line of the application execution screen in correspondence to the second length while the flexible display is extended from the portable device when the second length is equal to or less than the width of the main display. a main display; a flexible display; a housing unit configured to contain the flexible display; and a measurement unit configured to measure a first length of the flexible display, wherein the first length is a length of the flexible display that is extended from the portable device; wherein the portable device further comprises a control unit configured to: obtain a second length of the flexible display, wherein the second length is a length of the flexible display that is not extended from the portable device using the first length when the flexible display is extended from the portable device, compare the second length with a width of the main display, display an indicator indicating the second length only on the main display according to the comparison result, and move the displayed indicator according to a change of the second length in a direction in which the flexible display is extended or reduced when the second length is equal to or less than the width of the main display, and obtain a second length of the flexible display, wherein the second length is a length of the flexible display that is not extended from the portable device using the first length when the flexible display is extended from the portable device, compare the second length with a width of the main display, display an indicator indicating the second length only on the main display according to the comparison result, and move the displayed indicator according to a change of the second length in a direction in which the flexible display is extended or reduced when the second length is equal to or less than the width of the main display, and control the displayed indicator not to be displayed on the main display while extension or reduction of the flexible display is stopped, wherein, when at least one application execution screen is displayed on the flexible display, the second length is obtained by subtracting the first length from a total width of the application execution screen, and wherein the indicator is displayed as a virtual contour line of the application execution screen in correspondence to the second length while the flexible display is extended from the portable device when the second length is equal to or less than the width of the main display.