A display device comprising: a main display and a flexible display configured to display execution screens of applications, wherein a first application and a second application of the applications are set to be executed at the same time; a housing; a measurement unit configured to measure a length of a sub display and output a signal on the measured length of the sub display, wherein the sub display is a portion of the flexible display which is exposed to the outside of the housing; and a controller configured to receive the signal from the measurement unit, characterized in that the controller is further configured to: determine a size of the first application execution screen to be displayed on the sub display in proportion to a first length of the sub display when the first length of the sub display is equal to or greater than an initial value which is previously set and is maintained for a predetermined time, display the first application execution screen on the sub display, determine a size of the second application execution screen to be displayed on the sub display in proportion to an increased length of the sub display or a previously set ratio of the second application execution screen when receiving a signal for increasing the length of the sub display from the measurement unit after determining the size of the first application execution screen and the increased length of the sub display is maintained for the predetermined time, and display the second application execution screen on the sub display together with the first application execution screen display, wherein a ratio of the second application execution screen on the sub display is adjusted in accordance with the determined size of the second application execution screen on the sub display. a main display and a flexible display configured to display execution screens of applications, wherein a first application and a second application of the applications are set to be executed at the same time; a housing; a measurement unit configured to measure a length of a sub display and output a signal on the measured length of the sub display, wherein the sub display is a portion of the flexible display which is exposed to the outside of the housing; and a controller configured to receive the signal from the measurement unit, characterized in that the controller is further configured to: determine a size of the first application execution screen to be displayed on the sub display in proportion to a first length of the sub display when the first length of the sub display is equal to or greater than an initial value which is previously set and is maintained for a predetermined time, display the first application execution screen on the sub display, determine a size of the second application execution screen to be displayed on the sub display in proportion to an increased length of the sub display or a previously set ratio of the second application execution screen when receiving a signal for increasing the length of the sub display from the measurement unit after determining the size of the first application execution screen and the increased length of the sub display is maintained for the predetermined time, and display the second application execution screen on the sub display together with the first application execution screen display, wherein a ratio of the second application execution screen on the sub display is adjusted in accordance with the determined size of the second application execution screen on the sub display. determine a size of the first application execution screen to be displayed on the sub display in proportion to a first length of the sub display when the first length of the sub display is equal to or greater than an initial value which is previously set and is maintained for a predetermined time, display the first application execution screen on the sub display, determine a size of the second application execution screen to be displayed on the sub display in proportion to an increased length of the sub display or a previously set ratio of the second application execution screen when receiving a signal for increasing the length of the sub display from the measurement unit after determining the size of the first application execution screen and the increased length of the sub display is maintained for the predetermined time, and display the second application execution screen on the sub display together with the first application execution screen display, wherein a ratio of the second application execution screen on the sub display is adjusted in accordance with the determined size of the second application execution screen on the sub display.