1. A monitoring device comprising: an input device configured to receive an input of a plurality of images obtained by capturing a user and a sound source; a processor programmed to: detect lip regions of the user from each of the plurality of images, generate a dynamic saliency map of the detected lip regions of each of the plurality of images, acquire lip motion information using the generated dynamic saliency map, and perform voice recognition of the input sound source based on the acquired motion information; and an outputter configured to output a result of the voice recognition.