A monitoring device which measures numerical value information on a subject substance in a body fluid has an electrochemical sensor including a sensor unit for detecting the subject substance which is used in the way of being embedded subcutaneously and generating an electric signal correlating to the numerical value information on the subject substance, and a temperature control unit which adjusts the detected ambient temperature as a temperature ambient to a sensor unit when detecting the subject substance so as to reach a target setting temperature when measuring the subject substance.