A flexible device comprising: a sensor module; a flexible display; and a processor configured to: control the flexible display to display a first execution screen of a first application and a second execution screen of a second application in a first window and a second window, respectively; when it is detected that, by the sensor module, the flexible device is bent greater than a predetermined angle and such that the flexible display forms a front surface and a rear surface, wherein the first window and at least a first portion of the second window is displayed on the front surface and the rear surface of the flexible display is turned off, while the first execution screen of the first application and the second execution screen of the second application are displayed in the first window and the second window, respectively, control the flexible display to overlappingly display the first window and the second window according to a predetermined ratio, wherein the predetermined angle is an angle for determining whether the flexible device is bent or not; and when generation of an application switching event is detected by the sensor module, control the flexible display to swap locations of the first window and the second window and display the swapped first window and second window, wherein the application switching event includes a touch input to the rear surface of the flexible display while the flexible device is bent. a sensor module; a flexible display; and a processor configured to: control the flexible display to display a first execution screen of a first application and a second execution screen of a second application in a first window and a second window, respectively; when it is detected that, by the sensor module, the flexible device is bent greater than a predetermined angle and such that the flexible display forms a front surface and a rear surface, wherein the first window and at least a first portion of the second window is displayed on the front surface and the rear surface of the flexible display is turned off, while the first execution screen of the first application and the second execution screen of the second application are displayed in the first window and the second window, respectively, control the flexible display to overlappingly display the first window and the second window according to a predetermined ratio, wherein the predetermined angle is an angle for determining whether the flexible device is bent or not; and when generation of an application switching event is detected by the sensor module, control the flexible display to swap locations of the first window and the second window and display the swapped first window and second window, wherein the application switching event includes a touch input to the rear surface of the flexible display while the flexible device is bent. control the flexible display to display a first execution screen of a first application and a second execution screen of a second application in a first window and a second window, respectively; when it is detected that, by the sensor module, the flexible device is bent greater than a predetermined angle and such that the flexible display forms a front surface and a rear surface, wherein the first window and at least a first portion of the second window is displayed on the front surface and the rear surface of the flexible display is turned off, while the first execution screen of the first application and the second execution screen of the second application are displayed in the first window and the second window, respectively, control the flexible display to overlappingly display the first window and the second window according to a predetermined ratio, wherein the predetermined angle is an angle for determining whether the flexible device is bent or not; and when generation of an application switching event is detected by the sensor module, control the flexible display to swap locations of the first window and the second window and display the swapped first window and second window, wherein the application switching event includes a touch input to the rear surface of the flexible display while the flexible device is bent.