A monitoring device monitors deformation of a casing installed in a wellbore and housing a production tubing, and includes: a packer installed within an annulus between the casing and the production tubing; a deformable substrate that is disposed at an outer side of the annulus and contacts an inner surface of the casing to deform along with casing deformation; a light source that is disposed on the deformable substrate and emits light towards an inside of the annulus; an imaging device that is disposed in the packer to be opposite to the light source across the annulus and detects the light emitted from the light source; and a processor that produces a signal from the detected light, processes the produced signal, and transmits the processed signal to a surface control device that monitors the deformation of the casing based on the signal.