A portable device comprising: a flexible display unit; a measurement unit configured to measure an expanded length of the flexible display unit; a sensor unit configured to sense a user input; and a processor configured to control the flexible display unit, the measurement unit, and the sensor unit, wherein the flexible display unit has a first state in which the flexible display unit is not expanded and a second state in which the flexible display unit is expanded, wherein the portable device provides a dimming mode for dimming the flexible display unit when lack of the user input within a preset dimming time is sensed, and wherein the dimming mode includes a first dimming mode in which the preset dimming time is a first dimming time and a second dimming mode in which the preset dimming time is a second dimming time shorter than the first dimming time, characterized in that the processor is further configured to: provide the first dimming mode, when the flexible display unit is expanded to the second state, convert the portable device from the first dimming mode to the second dimming mode when the flexible display unit is retracted from the second state to the first state, if the user input is sensed within the second dimming time in the second dimming mode, revert the portable device from the second dimming mode to the first dimming mode, and if the user input is not sensed within the second dimming time in the second dimming mode, perform dimming of the flexible display unit when the second dimming time is reached. a flexible display unit; a measurement unit configured to measure an expanded length of the flexible display unit; a sensor unit configured to sense a user input; and a processor configured to control the flexible display unit, the measurement unit, and the sensor unit, wherein the flexible display unit has a first state in which the flexible display unit is not expanded and a second state in which the flexible display unit is expanded, wherein the portable device provides a dimming mode for dimming the flexible display unit when lack of the user input within a preset dimming time is sensed, and wherein the dimming mode includes a first dimming mode in which the preset dimming time is a first dimming time and a second dimming mode in which the preset dimming time is a second dimming time shorter than the first dimming time, characterized in that the processor is further configured to: provide the first dimming mode, when the flexible display unit is expanded to the second state, convert the portable device from the first dimming mode to the second dimming mode when the flexible display unit is retracted from the second state to the first state, if the user input is sensed within the second dimming time in the second dimming mode, revert the portable device from the second dimming mode to the first dimming mode, and if the user input is not sensed within the second dimming time in the second dimming mode, perform dimming of the flexible display unit when the second dimming time is reached. provide the first dimming mode, when the flexible display unit is expanded to the second state, convert the portable device from the first dimming mode to the second dimming mode when the flexible display unit is retracted from the second state to the first state, if the user input is sensed within the second dimming time in the second dimming mode, revert the portable device from the second dimming mode to the first dimming mode, and if the user input is not sensed within the second dimming time in the second dimming mode, perform dimming of the flexible display unit when the second dimming time is reached.