Abstract: Provided is dynamically converting a video being displayed on a user device according to changes in a holding direction and a display direction of the user device. The user device may include a processor: generate a request message for requesting a predetermined video to a contents server and transmit the request message to the contents server through the communication circuit, receive the requested video from the contents server in response to the request message through the communication circuit, detect a target object from each image frame of the received video and define a region of interest based on the detected target object; determine whether error occurs in detecting the target object at each image frame and correct the region of interest when error occurs; defining a crop region based on the corrected region of interest; and converting the received video to a second video based on the crop region.